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.