I have Real Solar System, Realism Overhaul and all the required/recommended mods for that installed, which means that all of my attitude control comes from RCS. When I try and use Smart A.S.S, it moves towards my intended attitude in about 5 degree steps, spending a lot more propellant than necessary, and taking a lot longer than it should. When it finally gets to my desired attitude, it's incapable of keeping it on target, and constantly moves around. I'm pretty sure this isn't related to having overpowered thrusters, as i get the same thing when using 220N, 33N and 1.8N thrusters, which I've verified are using their correct thrust using the mechjeb 'RCS thrust' stat. If I set MJ to use stock SAS, It's much more wild to start off with, but is far more accurate once it's in a few degrees of the target, but even then, it uses much more propellant than it should need to maintaining that attitude, despite there being no external force moving it away from that attitude, and it is incapable of operating with any form of physical timewarp.

ModuleRCSFX was merged into KSP some time ago

I'll have to rebuild an RSS/RO install to test MJ with it ...
I think one of the mod replace the stock RCS module with ModuleRCSFX. MJ may not be aware of the available torque from RCS and you get your behavior.
Can you check the attitude adjustment window for the "torque" and "|torque|" while RCS is online ?

Damn github notifications. Missed this. Booting KSP up now.

torque = [0.540, 0.500, 0.540] although the 540 ones flicker to 530, but barely long enough for me to read it
|torque| = 0.913

With rcs off
torque = [0.500. 0.500. 0.500]
|torque| = 0.866

Sarbian, that is correct - Realism Overhaul requires ModuleRCSFX and makes some adjustments there. I have also encountered the problem that OtherBarry sees.