Totally believable. I've clocked 50mph with 2 different '.12 powered nitros, albeit they were closed-wheel 1/10 inroads, and the 2.5 is a .15-size engine, I don't doubt that it could propel a lightweight vehicle like the Rustler to 47 or more.
You could use the nitro speed calculator recently added to the "tools' section to work out theoretical max speeds, I figure a 2.5 could do ~35k RPM un-modded on 30%.