I too am interested in this lens from an IQ perspective. I'm happy with the 2.8 II version, but if this is significantly better, I may be interested.
That said, dome requirements won't change from one to the other. The 9-ish inch domes will still be optimal since this is something that has more to do with the FL at the wide end than it does with the max F-stop.
Dimension-wise, these two are pretty similar; the F4 is a bit skinnier but about the same length, so port extensions may even be the same.
I can't wait for someone to do a comparison review underwater...... I'm certainly not spending money until then.... although from the topside reviews I've read, if I had neither lens today, I would get the F4 version..... Unless one absolutely needs 2.8 (topside), I believe the IS and lower price makes the F4 the better choice.