Bought a rear GTi bumper from GSF and wasn't impressed at all: the arms didn't line up with the apertures in the body. I'm not talking about being a few mills out, these were way off target! You could fit the arm in one side and the other was two inches away. Had to cut and weld to get it to fit, then discovered that the bolt holes didn't line up with the ones in the body.
Bought a front bumper that was a "pattern" part (I think it was from carpanels.co.uk IIRC, and it fitted perfectly and looked "proper" too.
Just my tuppence worth from personal experience, maybe other people haven't had any problems with them....................................