The UltraSPARC T2 has eight cores, each of these cores is capable of executing 8 threads; making a total of 64 virtual processors. The threads are grouped into two groups of four threads. Each core can execute at most 2 instructions per cycle, the load/store unit and the floating point unit are shared between the two groups of threads, but each thread has its own integer pipeline.
Usually, threads 0-7 are assigned to core 0, threads 8-15 to core 1 etc. The exact mapping is reported by the service processor (and prtdiag), but the mapping is only likely to be different if the system is configured with LDoMs. Taking core 0 as an example; threads 0-3 are assigned to one group and threads 4-7 are assigned to the other group.
To disable a core using psradm it is necessary to disable all the threads on that core. On the other hand if the objective is just to reduce the total number of active threads, keeping the core enabled, then best performance will be attained if the threads are disabled across all groups rather than just disabling threads within a single group. The reason for this is that each group gets to execute a single instruction per cycle, so disabling all the threads within a group will reduce the maximum number of instructions that can be executed in a cycle.