Post-cat O2 sensors

[Frodo]

I gather that 02 Sensor 2 (the ones downstream from the cats) are basically there to assess whether the cats are doing their jobs or not. If that's the case, if they (the sensors) begin to fail, will it at all affect engine performance?

I ask because I've had an intermittent CEL, the code for which indicates a problem with Sensor 2, Bank 2. Also, last time I had the car in to the shop the guy said his PIWIS scanner was indicating the Sensor 2 for the other bank was 'getting lazy'. The car runs pretty well but, it seems to me, doesn't seem quite as responsive on hard acceleration as previously.

I had planned on replacing both post-cat sensors, hence my question, ie: Can I reasonably expect any improvement in engine performance?