First off, cutting holes in a piece of metal can be dangerous.
Second off, if you have the appropriate tools, and know what you're doing,
it isn't hard (I've done that myself).
Now, the size of the particular fan isn't everything. You have to consider the fan's
speed and the amount of air per second that it can push through.
Now, obviously, if your 80mm and 92mm fans have the same speeds, a larger (92mm) fan
would be more efficient.
BUT, the ideal scheme is when the amount of air being taken in to the case and
the amount being taken out are same, as it creates a nice air tunnel inside the case.
However, all this steel-cutting stuff might turn out to be a fiasco if you're absolutley
unfamiliar with the procedure, as your newly cut side-panel might get bent or
otherwise screwed up, or you might cut yourself doing that, or lose an eye if not following
the safety rules when cutting metal with powerful electrical tools.
I don't mean to discourage you from this project, but it is your responsibility to
consider all possible outcomes.
Now, it all can be so much easier as long as you have a spot inside the front-bottom part
of the case where an intake fan could be placed, and if you have a predefined spot in the back
of your case where you could place an exhaust fan...and that without having to cut
anything. What's your case, anyway? Cause you just COULD place two fans inside of it, and
be a happy camper.
Just make sure you use identical fans and one is intake and the other is exhaust.