Hop up does decrease FPS from what I understand, but not by a significant amount. ie. on full hop, you might lose a few FPS but not enough to bring an outdoor gun shooting 395 FPS to CQB levels of play.
Hop up's main use though is to give the BB backspin so that it can stay on a straighter path for longer (thereby giving you a longer maximum effective range).
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE