My guess is, the "R" pack has different 18650 cells with a higher discharge C rating. If this is the case, it would make me wonder if the standard MX rated for 42HP is using the same electronics as the MXR which is rated for 50HP. My thinking is the MXR/EXR has a higher out put over the MX/EX due to less volt sag from the pack under full load. I am only guessing here, I have no idea if this is the case.
I agree, probably a higher C.
As far as the older packs being able to handle the higher current associated with the HP difference between MX/MXRs I'd need to do some wire diameter/power calculations. The main current carrying circuit between modules uses solid "rods" about 0.10" in dia. The negative return wire is also about 0.10" (AWG 10). Both terminate at the contactor which then connects them to the high voltage power connector on top of the pack (which looks to have either a voltage tap or current sensor built in) using similar gage wire. All wire connections are of the mechanical "crimp" type. If this circuit isn't enough to handle the additional amperage (heat/resistance) and surge loads it wouldn't be very difficult to upsize the conductors and/or use a more robust termination process.
The one area of concern might be the connection between the final Pos/Neg PCB traces and the Pos/Neg "receptacles" for the solid rods. These connections (8 in total) are all done interlayer. However, if necessary, external jumpers could be added to increase the capacity without much trouble.
I'd think the fuse wire connecting each battery in series would be adequate.