I dont fly the spit much, but IMO you should use the vertical as much as possible. The SpitV will munch the IX in a stallfight. Try to pull as few G's as possible at first, keep your speed up while going verticle on him. Keep the attacks fast enough though that he has to constantly react, make him bleed off his speed. Neither one has a guns advantage, both are fairly fragile planes, and both are fairly evenly matched in top speed and acceleration. The V has the advantage in horizontal turning only, so if you do find yourself stuck in a flat out turnfight, try to make your turns climb a bit. Use the extra power (not much) in the IX engine to gain some alt on him in the turns, then roll down on him when the opportunity opens. You have to make him break out of the horizontal turning competition, or he'll win.