Had a HKS SSQV fitted to mine and I did have a slight drop in MPG, also had flames at certain times I was told but I never saw em. Had no other issues but basically not having the standard (or an uprated recirc for that matter) will knacker the way the car runs, it looks for a certain amount of preasure coming back past the MAF on overrun and when it doesnt see it it just dumps fuel into the cyl's. Leaving it for a Very long time with poor servicing causes bore wash....not good.
I can tell you though you get so tired of it when your going through town, you quickly get bored of older people waving you down cause they think your a bus.
Now after saying all of that........................doesnt have sound the DB's when driving sensibly through back roads etc............... Rose so misses my dump valve, didnt speak to me for ages after I sold it ......... which was nice
Spooks