Im thinking of buying an Injen Intake but i want to know if a bypass valve is really necessary. I read somewhere that there is a minimal loss of power? If i do need it, how much would it be?

Not entirely necessary. The filter is pretty much enclosed by the bumper and plastic on the bottom... and the wheel well. In order to actually hydrolock it, you'd need to drive through about 2' of water.

is it only for tC or do tiburons(my friends) need one too?

I know nothing about Tiburons... sorry. It's possible that they're setup similar, but you'd really have to ask on a Hyundai forum... or maybe someone here has an idea, we can move this to All Other Vehicles to find out...