Shouldn't be. 100mhz is still 100mhz. Performance hit is possible if connection has problems and pcie drops to older version (pcie 1.0 instead of i.e 2.0) or lower mode (pcie x4 instead of x16).
Geforce 7xxx prefer +15 lod through the whole series of 3dmark except nature of 01 (needs adjusting somewhere in +3-5 range).
Riva will be able to correctly adjust lod with drivers up to 197.45 (maybe up to some early 25x.xx). Younger drivers are better tweaked with inspector.