Jed - the stock 4bts used in the vans made peak torque at about 1700 rpm. From an efficiency standpoint that is the best point to run the engine at for cruising down the highway. For max power you have to wind them up more, there is a thread on here that has dyno charts, can't remember the name of it now but I'm sure someone else knows. With a turned up pump and advanced timing the torque peak moves up some. As far where they will run reliably I'm sure they'll run fine all day at 2500, it just might wear out a little sooner than one spinning at a lower rpm.
Steve