Yeah, I have one. I don't know if it added any performance. It's designed to vent the excess pressure to the atmosphere when the throttle is closed, instead of the intake, so I don't know why that would increase power.
I originally installed the turbo by itself, and saw high intake air temps on hot days. (That was before they had the intercooler available). The blow-off spacer vents the excess pressure to the outside, rather than to the intake, so I added that to help lower the intake air temps. It seemed to lower them a little, maybe 10%. It also added the compressed air burst of sound like you hear on street turbos when they shift. Some people like the sound, I would prefer to not have it, but wanted to lower the air temp. I've since added the intercooler, which lowered the intake air temp a lot, and may put the original blow-off plate back on, if I get motivated to do it and can find the original plate. I've gotten used to the sound, so it doesn't bother me any more.
Here's what Banks claims for the plate, you'll see there is nothing about increasing performance:
I never noticed any turbo lag, so I can't say if it "quickened turbo response"
- Reduces intake air temperature
- Vents hot air to atmosphere
- Quickens turbo response
- Reduces pressure on compressor wheel during drop throttle
- Creates unmistakable sound