Orca can be decently fitted and boost to 442 with A-grade thrusters. It's no where near as maneuverable as a Cobra, but in a straight line, it can't be beat.
The Orca might have a slightly higher hard cap than the Clipper, but the Clipper can reach it's cap with a much larger useful load...same thruster class, less hull mass.
just tested a fully kitted out A rated courier, boost maxes out at around 375. Clipper A rated hits about 385. Didnt buy and try the others but im pretty sure a viper doesnt go up to 388 as it shines in its cruise speed, not boost speed. hmm
All of these values are terribly low.
My light combat Courier boosts to ~420, a well equipped Clipper boosts to 439-441, and a combat Viper 405-415, depending on specifics.
Data that I am using is from the wiki and may be wrong or outdated. Sorry about that.
Stock speeds don't mean much, and no one has bothered to put the speed caps for the given vessels on the wiki.
To hit speed cap you need to be using the best thruster the ship can mount and the total ship mass must be half of optimal thruster mass or less.
For example, the A6 thrusters on a Clipper have an optimal mass of 1440T, so a Clipper needs to weigh 720T total or less to reach it's maximum speed (441, which usually takes a double boost and some vertical thrust to see).