Yes, it's tourney legal. Both of my controllers have their springs removed. For me, it makes it a lot easier to shield/airdodge as I don't have to press as hard to push the button. This makes characters like Diddy Kong much easier to play as it's easier to perform L/R-based AT's like glide-tossing and insta-tossing.
Also, when taking the springs out, be sure not to strip the screws on accident. The screws on GC controllers are very soft so if you're too violent with it it'll strip the screw and you won't be able to screw it back in. I've taken my controller apart 4-5 times as carefully as I can, but the stripping on the screws is starting to show.
