> would I pay an arm and a leg for a P5020 based based system when
> a T10XX based board the would perform almost 3/4s as well

This may depend on the price of such T1-based system, I reckon.

> Even if SMP were supported the G5 would probably still come out on top.

This would depend on the specific PowerMac G5 model and the scalability of the task. With a task scaling well to four cores, four 2.2 GHz e5500 cores would be slower than four 2.5 GHz PPC970MP cores but faster than two 2.7 GHz PPC970FX cores.