125 MHz parallel bus in a double 2.54mm header
I think it should work. These connectors seem to have relatively low capacitance.

If you are really worry about this - you may try to pass 125MHz or even faster signal from lab generator and see what is going on oscilloscope.

These connectors were used for ATA hard disks, and they were working with pretty long ribbon wire with bus speed up to 33MHz, so without that ribbon cable there should be no problem with 125MHz (or 125MHz differential signal).



Source:

Upgrading and Repairing PCs
Scott Mueller

http://books.google.com/books?id=E1p2FDL7P5QC