So, if I were to compare a RM01 would it be to a Super Six or a Super Six Evo? BMC have 3 tiers of road carbon frames whereas most other brands have only 2 (not counting aero offerings).

Im not sure about the BMC Impec as its supposed to be their highest end model but the BMC RM01 is one of the stiffest dedicated road racing frames they make. Their new GranFondo is supposed to be super stiff as well but is aimed more at the spring classics. I think the BMC RM01 would be paired up against a Super Six and the BMC SLR01 Team Machine against the EVO. RM01 = very stiff, less compliant, slightly heavier. SLR01 = very stiff but not as RM01, more compliant/comfortable, lighter.

I think you are asking a subjective and almost impossible to answer question. Frame descriptions are largely marketing, and give almost no meaningful information. Listing different grades of carbon is less meaningful, because without knowing amount used and where, it doesn't specifically help. Rider weight and subjective feedback will be different, so the only comparison that matters is your own. Ride them all if you can, and answer the question yourself.

I had a BMC SL02 that did not impress me as far as stiffness, where others will rave about it.

Good luck.