Boost is only a tiny portion of whats going on vgt expands the usable range on a turbo but they do not negate a compound setup. If you have 25 psi of intake manifold pressure but 60 psi of exhaust manifold pressure performance well suffer one of the big advantages of a compound setup is balanced intake to exhaust pressure or delta. Older trucks pre vgt used a wastegate to help that but with vgt there is no wastegate. Also egr engines need a delta to flow if the egr is not used that delta just hurts performance.The new VGT turbo's pretty much negate any need for compound turbo's, my older Ecodiesel would hit and hold 25 psi boost, that's enough. The older Cummins engines in heavy commercial trucks only produced 21 psi boost, the amount of boost is directly proportional to big problems, the higher the boost pressure the more likely an expensive problem will occur.
sorry to hear about your experience, sounds like it wasn't really optimized for your truck. A 10% gain in power is very achievable. lets hear about your set up and maybe we can figure it out...Ran water meth for two years with no noticable difference in my intake manifold, even ran it so aggressive for a while I would go 8 gallons with every tank of fuel. At the end of it I came to the conclusion it adds 15 rwhp and helps lower egts a tiny bit but thats about it. Long term one is better off with a compound turbo setup.
I haven't seen a reliable compound setup on one of these engines yet. Larger single turbo would be the only change I'd make to the turbo.
Motorgoat811, what tank are you using for the -40 fluid?