Krusty, thats the thing.. the Spit16 is clipped-winged and has the same engine essentially as the 8 - the Merlin 66 (266 in the XVI). I'm pretty sure they have the same boost but I'm not sure. If so, the 16 should not outclimb the 8 at any time. I find the Spitfire IX to be most successful platform, but the climb is kinda smelly compared to the 8 and XVI, and an LF IX would be much better.
And I don't know about you all, but I feel uncomfortable flying a plane in this game when I know that my worst enemy has a better form NOT in this game. I.e. Flying a 109 and barely winning to a Spitfire IX would may have felt great, but remembering that there is actually a BETTER Spitfire IX out there (Spitfire LF Mk.IX with +25 boost) would make me feel funky... since it's left out and I'm shielded by the fact that it's not in the game. Besides, an LF IX without the boost would still kick a lot of arse. I've also heard that those +25 boost IX's turn super tight and climb super quick because of the extra power with no extra weight.
I'm not saying 'put it in the game', however... that is if I have no fellow whiners

To balance it out though, we could stick in the rear 75. Imp Gal. fuel tank in the Spitfire LF IX +25lb. boost... it would give more range but it would degrade initial climb and maneuverability. Besides, the thing would be found in the late-war arena where it's more or less competitive with uber planes like the 4-Hog. They've got in in IL-2, and, as good as it is, it still has troubles from Ki-84s and planes of the like.