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...........I agree with DragonAMG. I will go with Kleemann. The reason is that besides the fact that they have good products, they are by far the most user friendly of the list of tuners and tuner wanabees. If something goes wrong with your car, you can expect their constructive involvement.Also, their network of authorized dealers makes it easier to find a knowledgeable technician near you. There are much less expensive options especially ASP. For under $800 you can have an ASP pulley installed and gain about 35-45RWHP. The problem is that if you choose to mod your car further with headers etc, ASP has no further offerings. But if you were only going for a pulley upgrade, I think ASP product and price is hard to beat.
.....If you have a V12 twin turbo Mercedes, I'll say go with Renntech. Although other tuners have ECU/TCU upgrades for these cars, for whatever reason, only the Renntech cars seem to make it to the tack to demonstrate what they can do.
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.