But to get the C/R exactly the same you'd need a gasket that was as thick as a standard FTE gasket +2mm extra?
Not sure how thick the standard FTE gasket is, but for arguments sake say 1mm. Therefore you'd need a 3mm gasket to keep the C/R exactly the same as the FTE.
Surely the 2mm won't make a world of difference anyway, a small increase in C/R may be slightly beneficial in the scheme of things?