whoa, just noticed I never responded... what did you end up doing?
To answer your question:
Should we give the new subnet a different Vlan than the current (Server subnet)?
In general, yes, it's a good idea to have your workstations on a different VLAN than your servers. There are many reasons... one example would be... let's say one of your workstations gets infected and it's trying to infect other devices via broadcast discovery... well... the infection won't spread to your servers because they're in a different broadcast domain.
Personally, I use different VLANs for everything... workstations, servers, printers, wireless, management, etc. It makes auditing easier and can help you with deployment, etc if you start implementing things like SCCM. Although, this may also be overkill depending on your environment and what your objectives are.
My performance vs security comment had to do with where to terminate your VLANs (switch vs firewall).