Actually the Spitfire XII hits close to 380 mph on the deck. That's what the Griffon was optimized for in the Spitfire XII (but you have to check what kind of Griffon it is; some 12's were powered by a lower rated Griffon). Plus it climbes very well with the Griffon VI as opposed to the Griffon IIB. Also, it only does well at really low alt. The Mk.XII has a wingloading equivalent to that of the F4U Corsair anyway, and pilots are already giving the Spit8/9/16's some trouble out there in the F4U - the Spit 12 will probably just get mauled.
At any rate, i love spits, but i have to agree there's already too many to put in something like an LF Vb. Its a tremendously specific subvariant and none of the other planes have the oppertunity to have something like that, so neither should the spitfire. Also, clipped wings did not define the Spitfire as an LF. the 'LF' prefix was there because the Merlin engines used had their supercharger blowers cropped in diameter in order to produce a +18 lb. boost at low altitude. The Spitfire LF VIII (which is in the game) and the XVI can match every performance figure by 2.5K while having much better tops speeds. The Spitfire LF V would eat Zekes alive, but against BnZ fihgters, the only thing that the LF V will do is exist - or cease to do so if attacked.
Yea btw, the Hispano cannons on the Tempest Mk.V (series I / II) are fitted with 150 rpg. The inboard cannon was fed through a belt that was even more inboard, while the outboard cannon was fed through an outboard, curved belt so as not to have any funky guns setups like seen on the spitfire which had both belts coming into their respective cannons from the same side.