You can race it with rubbers no problem. Performance will not be exact the same as with foams, but still very fast.
In my country, one of the tracks has very low grip and foam works bad so at nationals events there you will see us racing rubbers no problem. In Brazil they have a parallel-to-national racing calendar all based on 100% rubber tires (alpharc being the dominant tire there).
I think in Japan and England they have rubber based nitro racing too ...
Here you have some rubber 1/10 nitro racing :
- Chile : http://youtu.be/mVV29FAWyAY
- Chile : http://youtu.be/FFC73wkIaGM
- Brazil (capricorn) : http://youtu.be/AmzqcMDEP6E
- Brazil (racing rubber class) : http://youtu.be/T7zJAjDcOVY
If you own a Kyosho R4 and needs rubber setup let me know man, national champion here race R4 and he already have a setup for rubber track as base start ...