I haven't had any performance issues so far, just preparing as we'll be using Quickbooks more often going forward (started a new business last month). The OS runs on an SSD, so disk latency shouldn't be much of an issue.
I suppose I can back up the VM, change the cores and see if it produces any problems. If it does, then I can just restore and decide later if its going to be worth it to re-install the OS. Upping the memory shouldn't cause any instability I would think.