Brainstorming: maybe make it mandatory only when a review is about performance of the game, because that's when it's relevant. Maybe a checkbox as you're writing a negative review that's something like 'what aspect(s) of the game don't you like', and if 'performance' is checked, a little addendum that says 'your specs will be added to the review to give context'. In that case, people who want to look through reviews would also be able to filter by those same 'types' of reviews, or see what percentage of negative reviews are due to performance, or other things. Could be a good addition overall.

But generally, if someone wants to write a bad review because of shitty controls, or plot, or the game being too short for its price, etc., there's no need to have specs attached to that kind of review.