where the lead can only show off his true strength in situations where he can get rid of all witnesses or if it's a true life and death situation. like legend of futian - he's juiced up with some rare emperor powers and if someone finds out he's basically enemy number one.. iirc.. haven't read it since it got stuck in an endless tournament arch which is boring as hell..
Not sure if it's what you're looking for, but To Be a Power in the Shadows! MC is OP, but tries to act as normal/weak as possible, except when he's in costume.