in my lab Back to back with 2801 IOS ver 12.4(24)T1 and two card HWIC-2T the following tests were doing:
*when i use the CAB-SS-X21MT and CAB-SS-X21FC cables
the highest clock rate that can be supported is 1000000 and lower
*when i use CAB-SS-V35FT & CAB-SS-V35MT cables
the clock rate supported is 8000000. "Fine"
there's someone to explain this result