Deleted member 1487
Sorry, wasn't aware that was the reason for the Mercury.I was trying to get more on the 'bomber' side of equation, thus the proposal for a weapons bay. As a pure fighter, the HS-12Y-powered, but otherwise OTL Whirly is certainly a better thing.
A 500 kg bomb was slowing down the Ro.57 by 35 km/h - from 466 to 431 km/h, or some 8% (all max speed figures). So we'd see a bombed-up ALT Whirly with V12 engines doing perhaps 520 km/h instead of 560+ km/h?
Reason why I've proposed Mercury engines is because we should be getting a surplus of those, since neither Lysander nor Blenheim are up to the task for 1940, so a major wind-down is in order for production of those A/C. A small 300+ mph dive-bomber or fighter-bomber will stand much better chances, but even then an escort will be needed.
On the other hand, a V12 powered dive bomber, size of Whirly, with wepons bay, might do 550 km/h...
Of course if you're more interested in the bomber part, then the Beau was available in 1940, just not in May. It could have used Merlins early on.
Though you might be better off with a single engine wooden fighter with a bomb rack to use the HS12Y engine like the French VG 33. That way you could have more flying for the number of engines even if using relatively small bombs; at least they could be a better harassing aircraft, able to drop a single 500lb bomb or so and dash.