I think it's not a matter of calculation because this would result in a higher latency as with Pre-comp size.
I see it this way. A normal compressor is a peak compressor and RMS size defines the amount of time which seen/treated as RMS and therefore not compressed. A kind of response delay.

 

I think RMS goes hand in hand with lookahead unless it's not going to eat any peaks.

that said, i downloaded those JSfx by JoepVanlier, will demo asap. when you try Geraint's JSfx you aren't going to believe how good they are, start with Atlantis Verb. the synths are dope as well.

 

Sorry for the ignorant question but where do I place these files? In the FXChains folder within Reaper?