It is fine for the sensor to be post cat. How do you think dyno tuners tune with the tailpipe O2 sensor? Also Allen Nelson puts his WB in place of one of the rear O2s when he is doing a live tune and I think he knows a little something about tuning

 

In the WBO2 shoot out thread Allen mentioned that he tried a bunch of different sensors in various locations -- pre-cat, post-cat, post-STS and saw no appreciable difference in the readings.

 

I'm glad you asked that question because I need to install my PLX and I was going to put it in the drivers side precat. But now I'll just but it after since my rears are deleted.
thanks.