Don't know about the Ultimate-II specifically, but:
- the MEGA65 already includes a Freezer Menu which lets you select the disc image. Currently .d81 images are supported (as the C65 had a built-in 3.5" drive), but 1541 support is also planned.
- the 1565 (==internal drive of the C65) was already many times faster than a stock 1541. The MEGA65 is even faster when accessing disc images. While not as fast as the SD card interface could be, it's on par with current fast loaders.
As for cartridges: Some work, some (currently) don't, but it's constantly being worked on. But I think once the MEGA65 is finished, there will be no need for the Ultimate-II cartridge.