Would it be difficult to cut with a straight edge razor?
Patiently, and with a sharp x-acto knife, it can be done. I don't know about a straight-edge razor.
Also if you don't mind, may I see a pic of how your subs look like with these mats?
(NOTE: While I haven't trimmed the mats under my subs - because I like how they look - I have trimmed other squares that I've used elsewhere. But I've discarded those squares, so I can't post pics of them.)