it seems to me that both you guys like your fast dogs ( denned mark ) both are 1/8 colie . so why not mate your dog denned to marks bitch.
you would then in theory still keep an 1/8 colie , and with marks bitch having whippet in there will probly bring the size down, you may also get a few brocken coats,
best of luck what ever way you go TOMO