Plan, according to what I remember from one of my Spitfire books (don't remember which one, I'll check this evening at home) the wing of the 21 was completely redesigned.
The original single-spar wing was deemed unsuitable for further developments, fact was that the new wing was not better than the old one (that was a design blessed by real genius)
Also the Mk21 had a very painful story of instability and bad behavior, that took quite some time to iron out.
I will check the book and provide quotes about this and an interview to a Supermarine test pilot who stated the lower handling performances of the late marks.
That does not mean that the XIV or the 21 (once the problems were solved) were bad aircrafts, but seems natural that increase in power, torque, weight etc. brought a faster but less pleasant and agile ship.