a 125 will hit 50ish
a 250 will do maybe 75mph at full throttle in top gear.
performance upgrades aren't really available for either model, as it's cheaper to just buy a bigger bike than mod up a small bike.
Honda's are excellent bikes - but these two are not made for highway use.
the MSF course uses 250 rebels, and they're really confidence-inspiring. I'd never take one on the highway, but they're great for around town, and easy to learn on.