What do you think of my buggy project?

[RC-Monster Mike]

The clutch has its pro and cons, just like anything else. It should smooth out the low end throttle response and "feel" a bit more like a nitro on the low end. It will also add an element of protection to the drivetrain. Cons....well, you need mechanical brakes, so more linkage, weight and overall clutter on the setup. The "softer" low end translates into less punch - this is an advantage in some areas and a disadvantage in others. No reverse, either - a racer doesn't need this anyways.
The extra mass of the clutch will be minimal or unnoticeable. This extra mass would typically yield more torque and peak current draw(flywheel effect), but the slipping clutch will minimize this to a degree. You may lose some runtime due to the slip (wasted energy), but in a 5 minute race this is a non-issue. The only thing you can do is try it and see if you like it. If you want to swap a little runtime and simplicity for the low speed "nitro feel", then the clutch and mechanical brakes is the answer. :)