The 914 front wing is shaped with a "S" like indentation. GT bumpers - like standard bumpers are straight edged.
As we are working with GRP I guess you could if you wanted to spend time and money cutting and fettling a GT bumper to meet that contour.
Again, it's something we talked about. But not for more than 5 minutes: time and money aside, GTs and standard cars never have that done, it would actually look "wrong" on the finished car.
The neatly fitted bumpers approach is something Porsche toyed with when they designed the 916. Nothing wrong with it, but somehow to my eyes it is simply too "sculpted" or moulded.

Instead on the straight edged bumper you line up the edge with the upper part of the "S", and allow a finger's width gap along the bottom part.
Again, only test fitted with tape, the fit will be neater and more uniform side to side later. And once painted black this will look very different too:

The lower valence will be lined up for optimum fit with the end of the arch flares. Very pleased with how they line up already, even before fettling:























