The autozone sensor should be fine. The SSAFC actually modifies your O2 sensor signal though which is how it is able to "tune" in closed loop under boost. Who installed and tuned your SSAFC? It sounds like your boost trigger for O2 override is set too low and it's modifying the O2 signal enough to trip the O2 signal. Either that or you are staying in boost long enough to trip the code.
The MSP comes with two O2 sensors, one at the manifold and one past the cat. As Spiced mentioned, the bung on the downpipe is plugged and a fantastic place to put a wideband. The first one is used for closed loop short term fuel trims. It is narrowband and can only tell the PCM your exhaust is rich or lean, there is no in between or magnitude read by the PCM. The second is also narrowband but is there to make sure your cat is working. The cat actually increases the oxygen in the exhaust gas by converting NOx gases into nitrogen and oxygen, but there is no amplitude reading by the PCM. However, due to the ridiculously slow sample rate of the PCM in the MSP (and most cars), the rich/lean signal from the secondary O2 will be slower than the toggle rate of the primary. Think of the O2 signals as sign waves, but the second one's amplitude is shifted lower. If you are probing both at the same time at a given interval, there will be times when the shifted signal reads lean when then primary reads rich. If they read the same for a long enough period of time, the PCM throws a code and says your secondary O2 or cat is bad.
I never owned an SSAFC. If I had I would have put an oscilliscope on the O2 output signal and solved the mystery once and for all. However, I spent an insane amount of time with the AEM FIC to understand how our PCMs and O2s work. When I offset the O2 signal enough that it was unable to reach a rich condition, the car would throw a CEL for a bad front O2 within a minute or two. It was actually was good to know that my FIC's offset was actually working.
I personally think the SSAFC and any piggyback is a band aid that could never possibly make our cars run perfectly. At least the FIC lets you see what is going on and gives you actual fuel control instead of just altering the MAF signal. Tuning our cars in closed loop is an incredibly arduous process that took me weeks of tweaking, so to think you can just thrown in an SSAFC and a forum tune and everything will be perfect is highly optimistic. I've read through most of the 100+ pages of the SSAFC thread and it's no mystery why tunes can work fine and then one day just turn to crap.
With any EMS or piggyback, it's almost essential to have two things: a wideband that you can data log, and a fast OBDII scanner to monitor close/open loop status and fuel trims. If you can data log the trims, it would be even better but all the readers I tried or possibly the MSP's bus itself was too slow to be of great use.
The simplest solution to your problem is to turn off your O2 modification in the SSAFC. You'll lose closed loop enrichment, but it probably wasn't ideal anyway.
