i own RPMs. i've tried an my 10m RPM back to back with my mates 11m C4 for about 15 mins in about 15-16 knots so ill only comment on lowend
C4 is definitely more of a C feeling, it was slower turning and had to be worked abit for lowend. Whereas the RPM seems a more grunty kite like a hybrid. i thought the low-end of the C4 was slightly better. they are both great kites. the build quality and attention to detail of the C4 is amazing
the 8m RPM is my favorite size. its a quick kite, so can be worked for lowend. i wouldn't say the boosting is particularly good (it may just be my skills)