the bodyshop i use have repaired 2 things for me. an original Greddy front lip and a not so original nismo front bumper. the greddy wasn't difficult to repair but it was ABS so different techniques are needed to restore rigidity and flexibility in the material.
the not nismo front bumper was GRP same as the real one. and the great thing about GRP is its a piece of cake to repair, and if done properly will still have the same structural quality and strength as the original (before break). if GRP isn't repaired properly and its done like a stick on plater then it won't be as good and will snap again.
the big thing to look for in any kit is the thickness and quality of the GRP used. as well as the fitment. most issues bodyshops see with cheap kits is partly poor fit and needing adjustment (but the same can sometimes happen with originals) but also paper thin GRP with bubbles in, it cracks and splits and needs a shed load of prep to get it ready.
i got nismo skirts from CS, fitment wasn't perfect, but then i factored that into the purchase at the time. but the one thing about the skirts from CS and the rest of the kit i had was the quality of the GRP. it needed very little prep (the new bits anyway) the front needed no adjustment.(had been previously fitted to another car). the bodyshop actually commented saying that the kit was very good especially as its a copy, when you compare it to some of the crap they've seen it was a relativly easy job.
i think i got my whole kit, sprayed and fitted for about just over £1000, i wouldn't have even been able to buy the full original nismo kit for that price, let alone have it sprayed and fitted as well.