I have no idea how they figured those barbettes were a good idea, in the design phase.
I don't get how the 410 would be slower than the 110 like that, below 10k. That blue line should have an indentation at 15k, considering it's there for the MIL curve.
As far as competing with mkXVI.. I'm not picking any argument here, but Krusty said it wouldn't compare and I don't remember anyone saying otherwise. A mossie or 1/2 ammo 110G will give a spitIX its money's worth, if flown right. I mean they have a much better margin to compete in than e.g. a 152, IMO. Whereas the 152 is restricted to forcing the spit to spill its E and then pound it with BNZ, the 110 and Mossie can easily compete with the IX on its own terms, for a few revolutions.
If you change the chart I posted above to include the basic 410 (2x50cal + 2x20mm) and put it in dogfighting trim (500 .50, 350 20mm, plenty!), it has better wingloading and powerloading than a mossie. It beats all the twins on that paper chart. Whether the estimated figures are accurate (this 131+151 is probably the closest, it's straight from historical docs) and whether they'd translate well in practice is another story (e.g. zero clues on departure behavior), but IMO it makes a compelling case for the 410 as a pretty viable dogfighter.

I'm not trying to play luftwaffle with paper figures. This is how it appears to me.