Is a 23x16 room too big for B&W 805S bookshelfs?

Yes they are too small, the bass will be very weak. If you plan to sit near field you will be fine.

Hmm... I absolutely love my current setup. Maybe it's time to step up to the 802Ds of the same series? Has anyone compared them to the 805S? Obviously there's going to be more bass, but what about the highs, mids, and everything else?

The effects or contribution of a sub in a large space will be relatively small. Setup and sitting arrangement are crucial in this circumstance. Listening distance from speakers preferably not more than 8' or you would lose the scale, enveloping sound and bass slam from the 805S ie. the whack of drums and the surreal feel of live dynamic music. A larger full-range floorstander will perform better but the 805S can sound decent if listening in near to mid-field arrangement.

I usually listen at about 80dba.

I suspect you may find the 805S to be borderline in a space that size. At least, I did in a space only marginally larger. It's the primary reason I sold my 805s -- which I loved -- and moved up a notch to the 804 Diamond - which works much better for me. But then again I didn't have a sub with the 805S.

I would fill the extra 120.5 cu ft. with music and a tiki bar, problem solved.

It depends on your taste in music and how loud you want to play; I would wait and try them and see for yourself.