Remass drives are naturally limited by the amount of mass and energy they can direct through their exhaust nozzles. (When I finally get around to writing it, my launch into space will be driven by a Solid Handwavium-based power supply so obscenely powerful that it can lift a 40' ISO container and assorted contents (over 3.8 tonnes, pre-'wave) with a "remass" of light, but once I reach L3 (after braking to stop at L3), the core Solid implodes like Doug played Who Made Who at it, taking the rest of the power supply with it. If I ever manage to reproduce the power supply, it will mysteriously fail unless it's powering a photon-remass drive and nothing else.)
For Speed drives and more exotic Acceleration drives, perhaps instead of a hard "top speed", above a certain speed/acceleration, energy consumption increases exponentially in relation to the rate, with both the "top efficient rate" and the terms of the fuel-consumption equation above that point being determined, at least in part, by the mass of the vessel. (In case it isn't obvious, this is an extension of JFerio's idea.) (There must be a clearer way of phrasing that, but I seem to have an abnormally low maximum cognitive power today and strained my brain just coming up with it in the first place.)
For Speed drives and more exotic Acceleration drives, perhaps instead of a hard "top speed", above a certain speed/acceleration, energy consumption increases exponentially in relation to the rate, with both the "top efficient rate" and the terms of the fuel-consumption equation above that point being determined, at least in part, by the mass of the vessel. (In case it isn't obvious, this is an extension of JFerio's idea.) (There must be a clearer way of phrasing that, but I seem to have an abnormally low maximum cognitive power today and strained my brain just coming up with it in the first place.)