1) There were only a few Mk.V's converted into Mk.IX's directly, but our aircraft, MH434, was built in '43 as a purpose-built Mk.IX. Our in game Mk.IX spitfire has the Merlin 61, which has better performance at high altitude than our Mk.VIII and XVI which have the Merlin 66 and 266 respectively. Down low, the merlin 66 and 266 (a Packard-built version of the 66), beat the 61 easily.
2) Our Spitfire Mk.VIII is actually an LF.VIII of Clive Caldwell of No.1 Churchill fighter wing in Australia. He actually didn't gain any kills while flying it.
Our Mk.VIII is designated 'LF' (Low-altitude Fighter) due to the Merlin 66 which is a low-altitude version of the Merlin 63, which in itself was a more powerful Merlin 61. Also, all Mk.VIII's were purpose built, and all 1,658 were built at Supermarine works. The Mk.VIII wasn't simply a more powerful and strengthened Mk.IX. It has shorter ailerons, a retractable tailwheel, and extra fuel. The two main tanks both hold 48 Imperial Gallons respectively (about 57.6 US gallons) while the Mk. IX's main tanks hold (top) 48 IG and (bottom) 37. The wings of the Mk.VIII also hold fuel. The leading edge directly inboard of the cannons hold about 12.5 IG each. Also, our Mk.VIII was a relatively common example since it has the 'griffon tail' which is the nickname given to Spitfires which had the taller, wide-chord rudder used on Griffon-engine Spitfires. All these mods made the Spitfire VIII some 200 lbs. heavier.
3) In real life, the VIII was found to be more stable but, if anything, it probably turned slightly worse at really low speed. In-game, our Mk.VIII is a better energy fighter due to the extra hp and weight (despite of which, the Mk.VIII still has a better powerloading), allowing it to dive a little better. Don't just turn and burn in the Mk.VIII. The Mk.IX, having a Merlin 61, wont climb, accelerate or run as fast as the Mk.VIII at low and medium altitudes as its engine only produces about 1,580 hp as opposed to the Mk.VIII's Merlin 66 of 1,720 hp at best alt.
The Mk.IX also has a similar boost rating to that of the AHI Spitfire V which, IMO was the best Spitfire V available with + 16 lb of boost. The Mk.IX makes + 15 lbs. of boost in this game but its more powerful engine allows it to perform better than the Mk.V, but not by a massive amount. Considerable, but not massive.