The 1.28g toilets from Toto work...people that have them are often surprised at both how well and how fast they flush. A few people have both the newer and older 1.6g versions, and find little, if any difference. the Ultramax II would seem to meet your requirements. Toto makes several seats for each of the round and elongated toilet designs. One of them is 'flatter' than the other, but I find either to be comfortable. The more dished one does force you to sit centered and back, but then again, that also means stuff tends to hit the water spot, too. Alternate seats can be used - the bolt patterns are standard on nearly all toilets (there are a few exceptions). Color matching is always a little chancey, and exact 'perfect' contour fit may not happen, but they work.