Hmmmm....
...I'm on my third attempt to answer. I'm not sure if the amount of thrust lost to the "vehicle's" gravitational pull would be significant, and not sure it wouldn't just scale up linearly to very large masses. The people who'd know are the ones who developed nuclear-electric propulsion and other microthrust/megatime strategies. I wonder if it'd even occurred to them to worry about gravitational effects on microscopic thrusts?
Hmmmm...(back where I started)