Interesting thread.
If you go with a wildly mixed set, you may want to change a couple of the saddles so that you don't get an uneven radius or playing feel. This will require a good amount of experimentation to get right.
There isn't that much of a difference between 9s and 10s, in terms of absolute string diameter. There are two significant advantages to using 9s:
1. Bending is easier (as mentioned above). The bit about legato lines is more mythology, improperly set-up guitars*, and lack of technique than sound proveable (and repeatable) science.
(* I've been SHOCKED at the terribly set up FRs I've got back from so-called 'professional' techs over the years)
2. When you bend a lighter guage string, it doesn't move the bridge as much as a heavier guage one. When you bend a string on a FR, the other strings detune. This bothers many guitar players, purists, and intonation freaks. I've found that good technique makes up for 90% of this problem, being able to mute the strings going down in pitch while keeping the ones you want to go up in pitch ringing out.
Now, as for my own opinion, I think that you get better tone from 10s, better tuning stability, and better intonation on fast runs. (Those tiny strings really flap around when you're playing fast) I've played FR-equipped guitars with 11s that were tuned down to D Standard, and I thought they played and sounded marvelous, a great way around the "heavy string - but still wanting to use the FR" paradox.
Anyway, that's my 2 cents, being an OFR user for abut 21 years now.
