As they say...In the end, you get what you pay for.

I believe this, because I've learned the hard way on previous cars long ago.
So with the SRT-6, I did my research, lots of research on many forums, most of which were MB forums and I decided to go with Renntech.
I bit the bullet and spent the $3k and needless to say, I couldn't be happier!

I've heard the SRT-6 puts out around 13 - 15 psi and Renntech says you get about a 3-4 psi gain...
3-4 may not sound like much, but it really is when it comes to HP.
My previous SRT-4 was pushing 25 psi on the stock engine with an aftermarket turbo, which put down 420 hp on the dyno.
When I would drop the psi to 20 for everyday use, it was a HUGE difference in performance.

I have no doubt in my mind that 3-4 psi on the SRT-6 gains about 25 hp and with the ECU flash which is tuned to run that 3-4 psi increase,
it could easily be a 35-40 hp total gain.
Im not worried at all about it hurting the engine either, 3-4 psi isn't squat considering the AMG engine in this car is built much better right
out of the factory with forged internals.
If my old Dodge SRT-4 could stand 25 psi on the stock block/head, Im sure this engine can take 18 psi just fine.

As for just doing an ECU upgrade, I would find it very hard to believe anyone getting more then 20 hp from it.
The car is tuned very good from the factory, as most AMG engines are, and the gains are just not there to be found.
When you add the pulleys though, you add boost, which adds more HP then any stand alone ECU flash could ever do,
but IMO adding pulleys requires an ECU flash to keep it running safe with the added boost, which will give you the best results.
That is one of the reasons why I went with Renntech, they tune the ECU to work with their pulleys/added boost.

Granted I have some other small HP add ons like plugs/wires and K&N filters, I couldn't be happier with the car...after all, I pulled off (4) 1.8 60 ft times at the track out of (5) runs on stock PS2 tires, all of which were 12.8 or better in the 1/4 mile with my best run below...
I think the numbers speak for themselves and proves the Renntech package is working, and my butt dyno says so too.