The DB603As make roughly half as much power at 30 kft. I'm not sure if I have info to make a powerloading comparison with other planes in AH at 30 kft.
and that's without a heavy Mk 103.
The overall numbers aren't flattering, but you've basically got powerloading halved (very roughly) from sea level power, and only ~10% penalty from a standard cfg (the basic 2x12.7mm + 2x20mm) to heaviest (BK5), and 5% penalty from 6x20mm or 2x103 to a BK5 loadout. So really not much difference across the board.
I'm thinking 410 because the expense in setting up a few passes seems possibly worth the benefit of the 410's extra firepower. As long as the B-29 is no faster than the interceptors considered, I think it's doable. I expect (been out of the game forever, so don't recall exact numbers, only overall trends) a 410 doing a few dozen MPH more than a B-29 is still manageable. It's only with really slow interceptors (e.g. B25H) that you really don't get more than a few passes and then get helplessly get left behind.
What the overall picture looks like is a lot like the standard Luftwaffe MO - only as much effectiveness from the plane as the pilot provides in discipline.
