Afaik the cpuid.coresPerSocket setting does not have anything to do with physical core usage, but only divides the assigned vCPUs and therefore just defines how the vCPUs are presented to the guest OS. Setting it to 1 has the same effect as not setting it at all. Btw, specifying dedicated physical cores (CPU Affinities) is something that should be used with care as it limits the CPU scheduler and could lead to less overall performance.
Regarding the CPU usage of your terminal servers. I deployed hundreds of virtual XenApp servers, and it really depends on the applications used, the number of users on each server and the hardware what works best. Although in most cases the limitation was memory, I had situations where 2 vCPUs were not sufficient and I saw a much better performance by assigning 4 vCPUs.
André