TGM did two really good jobs for me, albeit on the Quartz colour of my previous Z. I'm only not using them this time as I seem to have found a more local shop with a similarly strong reputation (mostly on Merc forums).
Unlike the poor OP, the scratch on my bumper is very small (two or three inches) and the guy reckons the location means he won't need to paint into adjacent panels (also getting a couple of alloys done at the same time as I kerbed one and nobody was confident of a perfect match unless both wheels on the same side are done - the body shop are sending them away to be powdercoated by Lepsons). I'll post up results in a separate thread next week.