I believe that unlike "traditional" FWK's, VSX FWK's run in user mode processes instead of being physically bound to kernel processes that run on each core. Given that, the load of these processes can be distributed and shifted across cores on the Gateway. This should enable you to grossly under or oversubscribe CoreXL cores to each VS instance.
If you are seeing heavy CPU load on a few VS's, but the Gateway itself sees overall low utilization, you should be able to add more cores without much concern. If there is plenty of CPU headroom left, I don't suspect you should see this change affect other VS's.
Another option could be to consider using VSLS mode; which will allow you to use multiple VSX Gateways Active/Active to distribute VS load. The only thing to keep in mind with VSLS is that you want to have enough capacity to survive a Gateway failure should one Gateway go down.
R80 CCSA / CCSE