On my rear I have a radial Dunlop 140/80, this makes the diameter of the tire about 1/2 inch bigger. Compared to stock, I've noticed a drop in RPM, and less power accelerating from a stop. On the plus side I can cruise on the highway at a relatively low RPM and I rarely get to 6th gear. However, acceleration is a bit sluggish and I have trouble getting it over 100mph.
So from what I've been hearing, it sounds like a smaller front sprocket will increase RPM (kind of) and counter the effect of having a larger rear tire. I think going to 15T sprocket should get me closer to stock gear ratios. Does this sound logical?
I figure at $12 for a sprocket, it can't hurt to try.

Your reasoning is spot on. 

Excellent.

A 140/80 Dunlop radial must be a D205 which is about 2" larger in dia than the OE 130/70.

Depends on tach accuracy, but assuming approx 4800 rpm @ 60 mph stock, the tire would give about 24/26x4800=4430 rpm.  A change to 15T front would give 16/15x4430=4725 rpm.

I have a 140/80 AV36 on the back of the old 97 GS with the 15T front and it's close to original rpm.  Haven't used it for a year so I don't recall exact figures.

Tire Dias:

OE Excedra 24.1
OE BT45  24.4
D205  26.1
AV36  25.7

The D205 is a bit larger than the other 140/80's.  I put those dias down from memory and just checked them.   

Seems like I ended up with the tallest of the tires.
What are some other consequences of that? Does it deform more in turns?

The 140/80 D205 is the same dia as the 130/90 tires and I've had three 130/90 tires that I put about 40k miles total on.  Touring type tires and the only one I didn't like was a K491 Dunlop Elite that broke loose too easy in corners due to hard compound.  Had a 130/90 Metzeler ME88 that I thought handled as good as the 130/70 BT45 it replaced.  My 02 came with a 130/90 Kenda K657 rear that I ran over 5k miles till spring when I put on Avon radials.

130/90 tire dias:

K491   26.18
ME88   26.2
K657   26.3

Ahhh,
Thanks for the info.