The difference in material doesn't make a difference, but the subtle differences in geos can create more variation than simply comparing effective top tube lengths. The different seat tube heights are not a factor. The seat tube and head tube angles of the RSL are slightly steeper than on the Tarmac, so for the same saddle position relative to the bottom bracket (height and set back), this will have the effect of adding some effective length to the RSL vs. the Tarmac, assuming that the effective top tube is being measured at the same height (which isn't certain). The RSL would appear to be a little higher in the front, since it has a non-integrated head tube height of 12.5cm v. 13cm for the Tarmac, plus an additional 3mm for the lower BB of the RSL.
If you plug all of the info into BikeCAD you could get a pretty good idea how the two frames compare. The two frames appear to be pretty similar, but to get your contact points in the same place you will need slightly more saddle setback on the RSL, and may have a different number of spacers, stem length, etc. If you take all of the relevant measurements off of your Tarmac, a Moots dealer should be able to mock them up on BikeCAD.