We have a customer with a 9000 sq. foot home on the water. Gas bill for all heating, cooking and hot water was 300 bucks for December. Condensing boilers cost more, but are worth it.
Regarding high velocity, everything I have read is you lose efficiency. My Lennox XP19 and variable air handler is 18 Seer, 9 HSPF. No way can you get that with high velocity from what I understand.