I found that it was hard to get good accelration with my Emaxx, it would just wheely all the time, and I still wanted some more top speed. So what i did was use a 2s and 3s together both are indentical and i added the center diff.
The 5s is perfect, noticable increase in top speed i even think I got a better run time.
The center diff, i'm not sure what oil is in there but in high traction the truck will wheely a small bit, but in loose or soft dirt / regular tires in sand the truck was much more controlable. I also have +20 progressive rate springs for slash rear, I only had to add one of the larger spacers and made it the same length as stock. these worked great I can hit some big jumps and as long as I hit the tranny no bottom out at all!