that's incredibly slow for such thin material and no use if you want to cut big blocks you will be there all day. With a cnc and no templates you obviously have to be very carful not to push the wire as if you do you will not get a straight cut when it comes to curves this is also where tension is very important so the higher the tension the less likely you are to get problems. I think with time even hard plastic would start to leave a mark on aluminium but by all means try it, personally I don't see it being accurate or stiff or durable enough.

If there is a slightly cheaper linniar rail option for the long axis I would be happy to pay for it the vertical axis could probably get away with maybe something like v bearings certainly if its only two axis but like you say you need smoothness or else it shows up very clearly in the surface finnish