: Is elaborating the opposite case in brackets acceptable and clear? Is Sentence A an acceptable and clear way of shortening Sentence B? Sentence A: A firm outperforms (underperforms) the

My Opinion

I have seen such a construct quite a few times in mathematical and physical papers, so I know very well of its existence.
Yet, almost every time I stumble upon it, it takes me quite a while until I recognise it, or the respective inversion is so blatantly obvious that there was no need to state it in the first place (or a simple vice versa would have sufficed).
I cannot recall a single instance where such a construction was actually useful and did not considerably interrupt my flow of reading.
If I peer-reviewed your paper, I would remark upon this.

That being said, these constructs are clearly tolerated in certain disciplines, whether I like it or not.

My Rationale

First of all, be aware that even though your intention with usual brackets (not the ones in question) may be to indicate that readers can skip the content within, they will not.
Instead, they will the contents of your usual brackets like an apposition, parenthesis, afterthought, or similar when first reading your text.
The brackets mainly have the effect to de-emphasise their content and indicate the sentence’s structure.
There are some exceptions from this like citations, but they are clearly recognisable as such.

A consequence of this is that your sentence should work with just the brackets (and not their content) removed.
You can also imagine the sentence being read out loud, with the contents of the brackets being spoken in a lower voice and a faster pace.

Now, if I read your sentence this way, it obviously doesn’t work; it’s not even grammatical.
This strongly disrupts my reading flow.
It then takes me some time to recognise that whatever is in brackets is the opposite of what was written directly before and that you are applying a special construct here, which requires me to read brackets in a completely different way I usually would.

On top, since such a structure employs at least two pairs of brackets, I have to pay attention to correctly associate them with each other and for example recognise that your “(1)” is not a bracket that is part of this construct.

Alternatives

If you really think that you need to be this explicit, rather write:

A firm outperforms/underperforms the industry when expression (1) results in a positive/negative value.

There is much not to like about this, but at least when you use a slash (/), every reader knows that what is following is an alternative, and you do not use punctuation (brackets) in a surprising way.

However, my guess is that when writing a sentence like this, you have already lost a few words on Expression 1 being a performance measure, and thus the sentence in question only explains how to interpret its values.
Thus, every reader with the mental capacity to understand your work will not need the brackets at all and you can just write:

A firm outperforms the industry when expression (1) results in a positive value.

There is probably no reason to expect that your measure will be the same for equal performance and underperformance, but if you really want to make sure, you can write:

A firm outperforms the industry when expression (1) results in a positive value and vice versa.

Sure, this requires a bit of thinking by the reader, but this is something they will think about anyway when reading these sentences.
After all, you do not appear to spoon-feed the following to the reader:

A firm performs as well as the industry when Expression 1 is zero.
The higher the absolute value of Expression 1, the more a firm outperforms or underperforms, respectively.

Finally, if your context is not fixed, I would write something like:

Expression (1) measures a firm’s performance in relation to the industry, being positive in case of outperformance.

This is as long as your original sentence, but contains all the information and much more.

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(0)

If you're certain you want to shorten the sentence in that way, the word 'respectively' would be be a very useful tool for clarifying what grammatical role is being played by each of the bracketed sections. In your case, I would write the sentence as:

A firm outperforms (or underperforms) the industry when expression (1) results in a positive (respectively, negative) value.

This is probably best used when the context is relatively formal/academic where readers are more likely to expect and understand this structure.

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(0)

If you are writing a paper in a context where the type of shortening in A is a recognized device for the kind of contrast you use in B, then you can use it.

Personally, I've never seen anything like it before—and I've worked on papers across a wide range of academic and technical genres.

To me, when reading the first sentence, it simply seems contradictory and confusing. It reminds me of somebody with a devil on one shoulder and an angel on the other shoulder, where the angel is speaking but the devil occasionally interjects with parenthetical opposites. In other words, the parenthetical words read like sarcastic statements, denying, or at least questioning, the validity of what's actually being said.

Had I not gone on to read B, I would have had to work at understanding what A was trying to express for far more time than had I read a simpler sentence. Brevity does not always equate to simplicity.

You save some words with A, but, unless you're certain that the device is known to the reader, you risk their confusion. Although I don't think B really needs to be shortened in the first place, there are other ways it could be done.

Such as:

A firm outperforms the industry when a positive value results from expression (1)—and underperforms when a negative results.

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(0)

It is acceptable, this kind of thing is done often in academic papers (to save space, there is often a page-limit in journal articles); but (B) is more clear than (A), that is probably why it was written that way.

Unless saving a handful of words is very important, stick with (B).

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(0)