Thanks for all the great replies! I'll try to clarify some of the items you all have mentioned.
1.) The Model 1 & 2's are not upgradeable to the Pro, as they don't share common parts other than the knobs. This is especially true of the Model 1's, because the optic bracket on the Pro does not have the correct orientation for mouse optics. FYI - mouse optics read movement vertically, so they have to be oriented such that the encoder wheel passes between them from top to bottom, unlike how opto-switches read movement horizontally across their path.
2.) The additional restrictor plates are next on the list. Best estimate right now is a couple of weeks, but don't hold me to that just yet.
3.) The Pro does use the same style optics as used on the Model 2, so the number of simultaneous spinners that can be connected is determined by the optical interface you are using, be it either an Opti-PAC, ME4, KE72-T, etc...
4.) DOT spinners... very much still a WIP. I have a few ideas, but nothing concrete yet. Besides, I just committed to getting the other restrictor plates out first.
5.) While I hadn't considered that people would want to purchase just the new encoder wheel, I don't see a reason not to offer it separately for those wishing to make their own spinners/steering wheels/whatever. If anyone is interested in purchasing just the encoder wheel, please send me an email. A word of caution, though... when testing this new spinner with a mouse hack, I found it to be quite susceptible to backspin. As I was not able to produce backspin with the larger opto-switches, I'm speculating it has to do with the tooth size/spacing, or the mouse optics themselves. I only tried it with the same Logitech mouse I use on the Model 1's, so perhaps other mice may not exhibit this behavior.
--Oscar