certainly improved lotsa hp in my case. Honestly, i don't know why the mkiv site has advised against this. I personally didn't feel any lag after the install. More importantly, you could boost a coupla psi more before detonation.
Having a FMIC will improve efficienty and is very helpful; if you plan to go further (upgraded turbos). I would be aware of the pressure drop from different FMICs though, search and try to find the best type for your needs.
This FMIC is one of the most efficient out there...check the thread out of ADFX's website ( www.ADFXRacing.com )