But is that because of the dish itself? Seems like the LNB would be much more of a factor. Maybe the LNBs back in the AT9 days were better made, and the ones made for the Slimline cut a few corners to save a few bucks?
Doubling the dish area theoretically increases your S/N by 3 db, so the ~4-5% bigger AT-9 dish would probably only help S/N by 0.14 db or so. That hardly seems likely to account for a noticeable difference in signal strength.
Are you getting 100s on any 99c/103ca/103cb transponders? 100s on the 101/110/119 are no big deal, but AFAIK 100s on the HD sats are pretty rare.
SL5, 3xSWM16, 21 H20-100, 7 H24-700/AM21