There is no downside to a full-frame sensor, except possibly cost.
I agree with Amir -- avoid teleconverters.
Thanks...i think for a novice like us, even if we blow up our photos to 4' x 4'...current technology at the consumer level is so good that this is entirely possible to do this, plus it seems to be more about the skill of the photographer anyway. 16MP is plenty of resolution with a crystal clear shot, and the lens gets us very close at 300mm and high quality vibration reduction...
so we will save the money, probably upgrade our 18-200 (27-300mm equivalent) and go for 18-300 (27-450mm equivalent)...and call it a day and get back to taking lots of photos of wonderful places.