Brushed motors, such as the type installed in a stock Stryfe always "burnout." Their commutators/brushes wear away and/or their bearings fail and they become useless. When upping the voltage and current sent to them, you will inherently reduce their lifespan.
So by using 2 or more Lithium (polymer or ion) cells, you will reduce the motors' life. By how much is pretty much speculation, but logic says you'll get shorter life the higher voltages you use.
Actually, ALL motors eventually fail, they just have different fail modes. Brushless motors can still have bushings/bearings fail, and vibrations will still work harden the thin wire used in the windings causing them to break at the commutator.
As for the OP's question, you are going to have to deal with the motors burning out eventually either way with a voltage upgrade, it is inevitable. The stock motors use flimsy metal brushes that will fail much more rapidly as the voltage is ramped up. If you want to hold this off, I wouldnt go over 2 cells (about 8 volts fully charged) from IMRs or a lipo. This is a minor upgrade though, and wouldnt give you the performance you would likely see from better carbon brushed motors (even at a lover voltage).