efficient setup for e-maxx

What I said above applies in general. After all, an emaxx is just another truck.

If you have to be able to do 40, I suppose that the 8XL will be the way to go. Having an emaxx doing 40 AND getting 30 minutes of runtime AND not overheat.

For all of your demands, I'd say go with a Neu 1515/1Y with a Quark Monster Pro controller (with proper cooling provisions) and an 8Ah battery pack. I believe that any setup using a Feigao motor that meets your criteria is going to overheat, unless you spend all of your time putting around.

Just FYI, most race setups are geared for mid 30s.