At our office we have already successfully done cutting to our Graphtec FC-5100 on a test 
Windows 7 Pro 64-bit system running Flexi 
Sign PRO 8.1, and Graphtec is generally known for terrible driver support and don't have anything for a 
Windows driver past 
Windows XP 32-bit.  If your plotter has a USB profile listed within Production Manager already, you should be able to use that to plot using 8.1, I believe.  So far, the only thing we have encountered with problems in Flexi 8.1 and 
Windows 7 is when printing to our Mimaki JV3-130.  We have not tried it actually, but have heard of several others having the issue so I foresee that as a problem.
In the end, we have decided to update our 
computer running Flexi 8.1 as a RIP to the Mimaki printer to Flexi 10 because the price was worth it to fix the incompatibility issues and still be able to run 
Windows 7 64-bit.  The upgrade for our PRO version was only slightly more than $300.