i'm currently not, but worked at a shop that built plenty of 500+whp wrx/sti's.
is going rotated an option? because if it is, i'd highly suggest it when you are aiming anywhere north of 450whp. It will be easier to work on which is nice, but the biggest benefit will be swapping out the oem style exhaust housing for one that will flow alot more, meaning the turbo will spool faster and make more power vs a stock location turbo of the same size.
I have an 18 sti, rod knock spun bearing?? rebuilt with an IAg closed deck block, fmic, flex fuel, ID 1700, radium hanger walbro 450, iag aos, bcp x500+, EL headers, up pipe ewg 3” catback, ported heads, gsc cams/valves/beehive springs. Pro tune session this Wednesday. Hoping to break that 500 mark.