Are you breaking in the motors before you run them hard? Running in water/dust/mud will decrease their lifespan significantly. It also helps to clean them after a every few runs and oil the bushings. I'm not sure what the stock gearing is the Stampede, but that's what you should run.
A 2s lipo will give about the same punch as the 7 cell(a little more in my experience) But you should only run lipo if your xl5 has lipo cutoff.
The xl5 is a "brushed". ESC, it can handle down to 15T motors. The VXL is the "brushless" ESC they use with the Velineon brushless motor.
Losi: 22T 2.0, XXX-SCT, Jr. T, Mini 8ight, Mini-T, Micro-T. HPI: Savage X E-Conversion. Traxxas: Slash 4x4 Platinum LCG, Slash 2wd LCG, Stampede. LaTrax: Teton. Axial: SCX10. Blade: Nano QX.