Hi, If you were torn between the GRP and a SPEED it'd be a harder decision As you've asked it, in my opinion its easy, BUY the GRP everytime !

I've had both, didn't mind the Vspec at all, average power, good run time, didn't like having to replace the rear bearing with less than a gallon on it, but knew it was likely to be needed when i purchased it.

The GRP is coming on for a gallon old and metal pinch is on the way out, run times are good, power wise it's really smooth with a strong bottom end but as yet it doesn't scream like my C6 does at the top (it's better than average but not awesome as yet). That may come with a little more fuel though her though ........ Build quality is awesome. I don't know what your talking about when you hint you don't think the GRP will hold a tune so well as the Vspec either .......... again it's been great

I don't like the GRP pipe,It's over restrictive, I find the motor alot more responsive with a JP-4 on it .....

Rod