Just FYI there is no legroom difference between the midbody (F/J/K) and the long bodies (M/R/acclaim/ovation). The lengthened body all went to the luggage area.
You also mention the longbody is 4-6 knots slower than the midbody. (shortbody is a C/D/E) I'm not going to stake my reputation on this one but I'm pretty sure its not slower. long bodies have 6cyl engines. the short and mids have 4cyl. Cruise speed in an M at 17k is 208ish. K is more like 190ish. You will probably need to stop for gas more often in the M so that might make the K faster overall due to skipping a fue