are you certain you have a k26/8 ? the spool lag (3k) sounds about right but the drop at 5k rpm sounds like a k26/6.
-water plumbing to to cool the turbo is good?
-intercooler is cold and well vented to keep cool?
-cat removed?
you mention MBC so i'm assuming you aren't using the dual-solenoid setup, which means as boost builds it will overcome the wg spring pressure and bleed boost at higher RPM. some people overcome this by using a higher pressure wg spring but that
over-rides the failsafe so i
highly recommend against this and suggest using an electronic boost controller with dual solenoid to both spool early and hold boost to redline. refer to this thread:
http://forums.pelicanparts.com/porsche-944-turbo-turbo-s/1048045-wg-tests-8.html#post11029395