From what I've read, the sluggishness of the Mk I. was due to the fabric covered control surfaces, which had a nasty tendency to inflate due to high speed air getting between the fabric and the airframe.
As for the Mk. VIII, I read that it was quite a different airframe, with several aerodynamic refinements, making it the easiest, sharpest, and most enjoyable Spit to fly. Truly the "sports car" of the Spits.
Daniel, aka Cyrano