Eric,
thank you for missing me...
Hope that all is well with you, Claudia and Sunny.
Now, to answer your questions:
1. Yes, the arpeggiator can be synced with an external source (midi clock); of course there are a few things to set in the Tyros 2 midi page as well.
2. The Motif ES has many sounds not found in the Tyros, and vice versa. Aside from the terrific acoustic bass I mentioned (which is sadly missing in the Tyros), I love many of the organ sounds of the ES, and find them better and more realistic, compared with the Tyros. On the other hand, the ES lacks all the beautiful SA voices, in particular the acoustic guitar and the tenor saxophone.
3. I would lie if I'd tell you that the ES is a piece of cake to program: it's indeed rather complex to set up and things aren't made easier by the tiny display. Luckily, included in the package there is an editor you can use on your computer to modify all the parameters (the computer must be connected to the ES via USB).
4. Unfortunately, the T2 has very limited master keyboard abilities; from this point of view, I think that the only arranger keyboard that can store settings like bank and program changes in a registration is the Korg PA1 X pro. It looks like Yamaha thinks that the Tyros is a self-sufficient entity, that does not need to transmit data to external devices. Roland did the same mistake with the G-70, which could have been a perfect master keyboard, otherwise.
If you are looking for an external module with very interesting sounds, to add to the T2 internal ones, I can suggest you the Fantom XR; I own one and am very happy with it. The arpeggios are not on the same level as the Motif ES (both in number and variety), but the sounds are different and complement the Tyros very well. Plus, it has EIGHT expansion slots, and Roland just released a card with killer electric pianos (the SRX 12).
Ciao!
Andrea