40 is quite do-able, just make sure you feed the system good batteries, otherwise the speed will soon drop off and the batts WILL cook with cheaper cells. Punch control and lowsest timing should make it easier to handle the truck, from a standstill, and reduce wheelies... in theory.
i just put a mm with feigao 8xl in my new emaxx with a 4cell lipo and it made me quite happy,cant tell you how fast but im thinking 35,wheelies on demand! and im running a rc monster 18 tooth spur,motor got a little hot about 140 so i think i need a 20 or 22 spur,hope that helps,love my emaxx now