Helo, Koopernic,
I too do feel that the B-26 would do better with greater power. The 2 turbos per A/C would add quite a bit of weight, however - around 2 x 800 lbs, judging by weight distribution of the P-47. The B-26 was already at the limits of the weight loading. Some weight should be cut by reducing armament and crew, but not all 1600 lbs?
The 2-stage R-2800 should offer better performance than turbo up to 20000 ft, and comparable between 20-25000 ft? Much smaller weight penalty, circa 2 x 300 to 2 x 400 lbs - easier to balance out with reduction of guns and crew.
The early B-26s were good for 325 mph, admittedly at not max weights?
Maybe install some fuel tanks in the aft bomb bay, or use some space where the navigator and radioman were?
The single stage engines in the Mossie were not specially tailored for low altitude. The Merlins XX, 21, 23, 31 and 32 were 'normal' Merlins, that gave better power at altitude than single stage V-1710s. The WER (over boosting due to ever better fuel) was only available at altitudes 'under' the FTH, though.
Interestingly enough, the single stage R-2800 did not received water-alcohol injection (ADI) until the 'C' series of engines, ie. too late for ww2. The turbo and 2-stagers were equipped with ADI from late 1943/early 1944. No ADI = no WER for US radials.
The V-1710s were allowed for WER without ADI (= WER dry), officially from mid 1942 on. Depending on version, altitude and time, the WER was between 1400 and 1600 HP.
Seems like the Germans embarked into the 2-stage bandwagon too late. Big engines, decent RPM, intercoolers and MW-50 should enable good high altitude performance even on the B4 fuel.