I run my S12 on 15psi and its fine.It did 187,000 on stardard boost.(Mileage when I bought it)
If i run her past 5500rpm it does burn a bit and black smoke visibly comes out the back end.
Thats the rings because i did the the head when i got it and its a recon t25 so it must be the rings i suspect.
My emissions are a bit iffy but i know the place that does my M.O.T so im ok