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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