I love my Renthal 758s, too but the Spanish MOT dude doesn't. Unless it has an EU type approval certificate, he won't accept any aftermarket part fitted to a bike these days.
Oddly enough, I e-mailed Renthal yesterday asking for just such a piece of paper and they tell me that there's no requirement for one in the UK, so they don't bother with it. So, how come they can sell in Germany (TUV rules) and elsewhere within the EU without type approval?
Beats me but if anyone knows where I might find the necessary piece of paper - maybe one came in the package with your new Renthals - I'd really be grateful for the pointer.