I have an 03 VIII that I have noticed that it feels good at partial throttle, but the more I lay into it the more the throttle response goes away. I have chased a few leaks out of it (BOV, throttle body, boost gauge intake patch), and it makes good boost #'s (18-20.5 psi, depending on the air temp, it likes it cold & dry). I can hear the turbo spool up and whistle, with a good amount of BOV sound. Going to WOT has not much get up and go; it feels better at partial throttle. There's not much of a top end hit.

Plugs are recent, it has a K&N drop in and mail in Dynoflash as the only mods. I've been chasing this one for a while but haven't been able to pin it to anything. There are VERY few Evo's in my area so I haven't been able to compare it to another car.

 

The mail in probably doesn't match your car. Try to get an A/F meter and check that it's not too rich on higher revs