I had a Hip-shot tremsetter, and found that it was quite easy to go to drop-D simply by detuning the fine tuner on the low E. It worked flawlessly, returned to zero every time, and I could still operate it fully floating. It's a good unit. The performance will be the same if you get one and set it up properly as I did. As for wether or not you'll need a D-tuna to go to drop-D, that's entirely up to your preference.
You'll need to realize though, that a unit like the tremsetter is ideally essential to work in conjuction with a floating trem if you decide to alter tunings on the fly. It's not called a "stabilizer" for nothing. When you change tunings, you alter the counter tension of the strings against the trem springs. They will always want to equalize in their tension. Without something to make it go back to that "zero" position when you've altered the tension, it will want to sit where the tension is equal. Simple laws of physics applying here.