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Non-turbocharged P-38s may have been better for sheer speed as well as fighter/bomber duties, but I don't think we need to go over production delays and shortages with that type again. (it was covered rather well in the discussing regarding producing more P-38s in place of P-40s and P-39s -even if the value was seen for an aircraft nearly 2x as expensive, the production capacity simply wasn't there early enough -unless the XP-38 doesn't crash and they gain a year in development along with a properly optimized unturbocharged version with counter-rotation, ejector exhaust and ram intake optimization on par with the P-40/P-51)
Get Vultee to produce that (non turbo P-38), instead of the Vengenace - per gjs' question re. Vengeance's future?
I'm not sure the Defiant is that worth being written off, but development limited to turretless operation would be the focus for sure. The larger, thicker wing of the Defiant may have been more suitable for cannons than the hurricane while still having overall drag characteristics better than the Hurricane and possibly better than the spitfire considering the weight and wing area. Maybe a better candidate for fighter-bomber conversion than the Hurricane (especially with the Griffon), but the hurricane still had advantages in manufacturing infrastructure that kept it in production so long historically.
If we really want a cannon-armed fighter powered by 1-stage Merlin, stick two cannons on the Spitfire as historically Four cannons on a single-stage Merlin means quite a big drag weight. Hurricane also have had thick wings (and of greater area than Daffy), maybe even thicker than Defiant. Thin wings were the key to Spitfire's performance, Spitfire was smaller, the Daffy was with wider fuselage because of the turret, so it would be hard pressed to emulate Spitfire's performance on same engine.
The Defiant have had fuel tanks in the wing, those should be relocated in the fuselage (assuming the Daffy is now 1-seater), the weapon bays with extras will be needed to have the guns/cannons installed.
In any case, single-seat defiant derivatives may have had enough merits to not simply throw them away in favor of Spitfire production.
Single seat Defiant is a recurring theme, I've even sqetched a long-range fighter based on it. However, once we recall how even the Spitfire was in performance disadvantage in a good part of 1941 and 1942 vs. LW, having yet another aircraft that has 10-20 mph disadvantage does not much good for the RAF and the Allies.