Amount of GHz is only important if you have applications that rely on pure perfomance / clockcycles.
As you mention yourself, make sure to understand the number of vCPU vs pCPU's, ready times etc. this is the major fault which causes performance problems.
If this is right, and you still have noticeable performance problems which relate to CPU, it could be the amount of GHz.
For example you have a VM with 2 vCPU which uses 100% CPU and the application/amount of calculations is still is too slow, you could try processors with more Ghz, these will cause an direct increase in this.
From my experience we have 1-2 of 1700 applications which are directly related to the amount of Ghz for calculation throughput.
But your choice for 2,5 GHz will be good for most use cases.
The total amount isn't relevant indeed, it's just a calculation of the total amount. As soon as the ESX scheduler schedules your VM, it will get the amount of vCPU with the amount of GHz from your hardware.
VM = 2vCPU
So when it runs, it claims 2 x 2,5 Ghz to run.
Hope it helped a bit in my crappy explanation :S