First upgrade you should make is to go LiPo and ditch the stock NiMh packs. Get yourself a couple of pairs of 2S 5200 Turnigy packs, a charger, balancer, LVA and a go for it. This will increase the run time. I get 20+ mins on an E-Revo with a pair of Kershaw 700HO's so a stock E-Maxx should run 25+ mins.

Next I would swap out the stock titans for the Kershaw 650 or 700 setup. The 650's are a direct bolt in as the 700's require a new motor plate.