@Thomas - I thought they were spun at half the rate because of the Paula, not Agnus?
OP - AFAIK an Amiga high density floppy will be "plug-and-play" on an A2000. Probably needs 3.1, but I think you've got that covered already. Other companies made high density floppy drives for Amiga's back in the day (Applied Engineering comes to mind), but those drives did require a software patch and you couldn't boot off them (that's just from memory).
Edit - just googled up this thread: http://eab.abime.net/showthread.php?t=55344 Lot of information here.
I have an Applied Engineering High Density floppy drive. It is basically a HD mac floppy modified for use with the Amiga. It came out in the OS 1.3 days. Like Oldsmobile_Mike said it needs a softwave driver to use HD mode, and therefore does not autoboot in HD mode.
Unfortunately the driver does not work for OS 2.0 and above.
On the upside the drive is extremely well built and has a really nice auto-eject feature like most mac drives of the generation.
Additionally the HD mode format (only 1.52 meg formatted) is not the same as Amiga HD floppy format (1.76 meg formatted). It is to bad that the amiga cannot be used with Amiga HD floppy Format disk, nor is there a driver for OS 2.0+.
