well one thing you gotta keep in mind is that the tighter a binder is, the less suitable it is for sports.
underworks binders are usually worst because they are long, even the tritop is rather long and so is the binding area of the 988, and hence they constrict your ribcage more than they'd have to. a binder you wear to the gym should be as short as possible. now there's two ways to achieve that: you can cut a regular underworks binder down to the length you really need. this only works with underworks or DANAE binders because the material can be cut in any place without losing shape and binding power.
or you can buy a shorter binder. i put up a list of short binders and comfort in some other thread, i guess it must have been the "the most comfortable binder ever" or the "binders and comfort" thread. but i created that list based on overall comfort, not so much on whether they constrict your lungs or not.
long story short, i would go with either this one:
http://www.lesloveboat.com/shop/product_info.php?cPath=52_56&products_id=202 ....which is super-comfortable, but get a size smaller maybe, because i got mine the correct size and had it let in at the seams. i can take full lungs of breath with it, it doesn't cut in at all, the material is super-soft. this is not the thinnest binder though, but it's supposedly breathable. it's the binder i would most likely go to the gym with but it probably won't get you super flat.
or this one:
http://www.lesloveboat.com/shop/product_info.php?cPath=21_34&products_id=39this binder has velcro on one side so you can adjust it so you can take full breaths. it quite breathable and it binds really really well, so if you're looking for a binder that also does the job outside the gym, this is probably the one to go with. it's also short enough not to compress more of your ribcage than necessary. the outside is cotton, which i actually prefer since it won't trap sweat the way synthetic fibers do.
