Your motors are most likely wired in series, which is why the mechanical speedo hasn't blown up yet. If you rewired the motors the preferred way (in parallel), an XRS still wouldn't handle it. But it would probably work the way it's wired now. Even so, you're better off with the Mamba Max or look for a Super Rooster or maybe the Traxxas E-Maxx speedo (made by Novak) on Ebay.

However, if you swap those stock silver can motors out for a couple 55 turn lathe motors, you could get away with the XRS. But the money your trying to save buying the XRS is negated by having to buy new motors. But those lathe motors will make a huge difference in it's crawling ability.