Thicker grease could help a lot - I use very thick, pink diff grease for the main diff plates (Sold on oOple some years ago,
LINK - If I recall correctly member madmax (Lee) handed me a batch of this grease at the 2012 Kampenhout GP. I'm still using it for my diffs today, thanks Lee
) and Asso black grease for the thrust bearing. Is it also worth having a look at Tamiya's AW-grease?
For reducing ballooning, you could also try moulded inserts, or stretching tires. For example a 4wd tire mounted on a rear rim, or removing the outer ribs of the rim and glueing the tire to the edge of the rim - suggested to me by member janus
Stretched carcasses will not only reduce ballooning, they also sharpen up the steering with more predictability and a more confidence inspiring feel.