Never heard of a 1.3 meter, but that doesn't mean they aren't.
The difference in gain between a 1.0 and 1.2 @ 12Ghz from my Channel Master/Primestar notes:
1.2m = 48x52 3537.5573376 40.70 dBi
1.0m = 40x43 2456.63704 39.12 dBi
I just happened to run the numbers for the 2.4m that Cham recently picked up. Like 51db, YIKES!
db figures are at 50% efficiency. Parabola Calculator 2.0 used.
All other things being equal, isn't it fairly obvious that 1.2 < 1.3 <1.4 <1.5 < ... ?
You're talking "spill-over" from Mexico? I've received perfectly watchable signals from Central American footprints here in Oregon using a 1.2m, specifically on 113W and 116.8W, although admittedly I've had to go manually push-tune the dish for optimal reception.
I haven't had my 1.2M dish up for while but I don't think there would be much difference for and it wouldn't be worth "upgrading" even if you could find one.