If you are still on 6mm belts, then the 9mm belts are a slam-dunk in terms of value — get a maintenance kit (be sure to e-mail in to shipping@carbide3d.com when placing the order to note that you have 6mm belts and will need new pulleys) — and you can upgrade to 9mm belts. Some folks have had good luck w/ belts using materials other than fiberglass.
The larger SO4 V wheels won’t fit because there isn’t clearance for them — same goes for the 15mm belts — the only way to replace these would be to replace the motors (you’ll need longer shafts) and design new endplates which would have suitable holes for the new V wheels, and you’d need to drill new holes in your endplates — at that point, you might as well sell the machine and use the funds towards buying a new one.
What spindle are you using? We should be offering a VFD spindle upgrade in the near future.
What materials are you cutting, using what tooling at what feeds and speeds?
What CAD/CAM tool are you using? Have you looked into adaptive/trochoidal toolpaths?