My take is that with non-turbo V-1710 the gain is practically nil - just another plane that's good for combats under 15 kft, but a bad choice for combats above that alt.
Now, replacing R-1830 with V-1710, in P-43, offers some better performance, at least on paper. P-38F was served with 1325 HP at 20 kft, 1240 HP at 25 kft, in second half of 1942, so we can use that power for 'our' P-43. Such power was comparable with Merlin 60s, but those can go to WEP (or 5min regime?) for more power, albeit at lower height. The P-43 itself was Spitfire-sized, but it was heavier than Mk.IX, let alone the Mk.V. The armament need to go to 6x .50 cals, with at least 350 rpg. Protection also need to be more substantial. That would mean some 500-1000 lbs more for take off - 8000-8500 lbs 'loaded' weight? Heavy indeed, but still some 10% than P-51D.
My take is that top speed would've been something between Spit V and IX (390 mph at 25 kft? - turbo cancels the exhaust thrust mostly), with climb comparable with P-51D, available from late 1942 on. That's with P-38 engine power for era; from late 1943, it's 1425 HP MIL (400 mph +?), and 1600 HP WER (410-420 mph?).
Only US plane that was really fast at altitude, prior second half of 1943, was P-38. Splendid plane, but not that available, with many bugs to sort out. A more affordable plane, to bulk out the numbers, but of similar performance, available a full year before P-47, or 15 months prior P-51B, would've been very good for Allied cause. Of course, providing it was offering a substantial internal fuel capacity (218 gals - non-self sealing in real P-43?); 2 x 108 gals drop tanks at such a plane would've take one further away than when attached at real P-47.
Joe, XP-47A was featuring V-1710 in the mock-up IIRC.
