In a standard environment with modern hardware, CPU contention issues are now very rare. the major bottlenecks particularly with VDI are Memory and Disk IOPs.
your current environment hosts 225 guests, even taking into account ESXi overhead you are still not in a contended state. the addition of a futher 150 guests will take your contention rate to approximately 1-1.2 this is next to nothing.
so long as your Guests do not currently exhibit high CPU utilization you SHOULD be fine, I say Should because I have no evidence regarding current Guest CPU utilization. if your guest are regularly running at 90% CPU then the additional 150 desktops could be too much.
However if they are running at say 25% CPU Utilisation then you could possibly install 500 more guest on your environment without major impact.
My Gut Feeling is you will be fine. however please do your homework first by testing the resourse utilsation of your hosts and Guests over a set period, hopefully taking into account peak usage periods like Month end.
