I don't know what the ratio is for 1st gear in the M90 but in the M46 it's pretty short (>4:1) so even with a lighter flywheel I doubt you would see too much drawback in driveability (as far as step-off). With that really short first gear you might get some improvement in acceleration off the line too. However, I doubt the difference would be very large, and unless you're planning to do a lot of stoplight grand prix stuff, having a heavier flywheel will probably help the car be more comfortable to drive most of the time. Particularly the case if the M90's low gear is not as short as the tractor setup in the older transmissions.
Are you planning to change the rear end ratio as well? In the US, ZF cars came with a 3.91:1 R&P while M46 cars had a 3.54... Dunno what was the original arrangement for the M90 diesel cars but if their gearing is anything like an M46, I think running with an automatic 3:91 rearend would not be too pleasant... 1st gear is already almost useless as it is, and the motor spins 2500+ at highway cruise. I'd probably want to change to a 3.54 at least, or maybe better yet a 3.31 out of a manual trans gasser...
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