The main issue is that in order to use jumbo frames you need to have support for that frame size in all the devices in the same subnet. So if you want to have your (presumably) non jumbo-supporting older computers in the network also, you need to have them in a different subnet and route between the subnets.
Okay, so if what you're saying is correct, then it's a "weak link" problem where one non-jumbo device on the subnet will spoil it for the rest. Maybe the same thing is causing my 1300 problem--i.e., some device is capped at 1300, and propagating this limitation to all of the other devices on the subnet? Could it even be the cable modem upstream of the router doing this?