FYI, I've driven manual steering Miata's, and they do not turn as well as the power steering one I eventually bought. In anything longer than a 5-cone slalom I can see getting behind easily and just plum wore out, especially on big sticky tires. If your open lapping/HPDE'ing as well I would definitely recommend keeping the PS.
I got some touring laps on PIR with the Miata yesterday and loved it. The feedback and feel from the PS was excellent and I could honestly say that I could drive around that track all day and still be sharp at the end of the day. The drive home from events is infinitely better with heat/A/C/PS/CD as well...

