a few on the site have...more current flow= more power is the claim and some say good others a waste. If you have a system it will improve battery charge is the only real improvement I would assume. Dynoed I think it yields 1hp at max. It is a good way to improve everything in the charging/running system, but in the end thats all it is. Don't spend your money tho, if anything get some wire and terminals and do it yourslef for less. Def ungrade battery ground to atleast 4 gauge, the stock I've heard is minimal and could be much bigger. Hope this helps man.
I can tell you it will smooth out your engine. Provide better acceleration. (Less stumbling)
I purchased 4gage wire at PepBoys. Just get the black negative ground cables, they are like 4 bucks a piece and come in various lenghts. I ran one from the battery to the Block near the Fuel rail bracket. One to the body and the front corner of the block.
It doesn't make noticable difference in performance, but it will fell better.
Just cleaning and regrounding the stock connections will get the same thing. All a "special" grounding set will do is to put you back to where the car was when it was new. It's just another product that takes money from your pocket and puts it somewhere else.
If you car has grounding issues this will help, but liek everyone said just pick up a roll of 4guage wire and some decent ring terminals and get to work. I personally upgrade the main ground to the battery, Add and extra ground off the alt (usually see a voltage increase there) and maybe 1 or 2 engine grounds if there looking bad. The extra voltage helps a bit with the charging and running of electronics.
And remember the better the electrical system in your car is the less your altnator needs to work, the less restiance on that means less parasitic loss, which in the end can mean 1-3 hp in some cases.
On my 91 tsi i did the grounding over, got 1.5v extra charging voltage and it did help with my blowing out spark issue somewhat by giving the coils more juice.