I don't think it is possible to swap the 2,7 GHZ modules to a 2,0 GHz mobo and get it working. I've succesfully swapped 1,6 GHz mobo with 1,8 GHz CPU and 1,8 GHz mobo with 2,0 GHz module, but nothing more.

I've heard that people seemed to use the air cooling with 2,7 GHz CPU modules, but I guess the thermal calibration would be broken then (loud machine) unless you'd use some non proprietary fans.

You could try doing the PRAM reset procedure, but I don't know if that would help.