i would never ever ever "upgrade" to a hta68 over a vf.
unless you're talking about a td04 equipped 2.5L, but even then just get a used vf turbo for $400 and call it a day.
I have an 02 wrx and an 06 sti and been through many setups on both and i wouldnt put anything less than a 49 lb/min turbo on either of them.
although i finally got the td04 back on my wrx to try and break the 12.4 sec 1/4 record.
I only did 1 pull.....made 220whp at 12psi on the road dyno. hoping to hit 300whp at 20psi.