I ran a HKS on my car for a while...
Appeared to run fine at first, no apparant loss in power, no stalling etc... though the fuel consumption suffered...
After a while though, I started to get limp mode... at every gear change!! Very annoying driving a 1.8t in limp mode... Vag-com showed a few faults as well...
Recirc valve went back on and not had any limp mode since...