You should set at least 2Gb using "-Xmx2G", but given that you have a lot of memory you can go higher. Just experiment a little. Please let me know your results.
Could you also describe in more details how you work with 200+ pages? Do you have them all open at once? What kind of operations you are performing that are slowing things down? It would help to know your exact scenario so that I can check for any possible optimizations on my side. Perhaps you could even send me your project (you can export your project into a zip file) so that I can experiment with it locally. Thanks!
You can email me a zip file with your project at firstname.lastname@example.org. Zip file can be exported directly from WireframeSketcher. Right-click on the project in Project Explorer and choose Export > Archive File.
Also it would be helpful if you could describe briefly the steps for reproducing the performance issue. Thanks!