yes, if you're running catless and you throw a CEL, you should more than likely be fine. but just to be on the safe side you should have some sort of data logger/scanner to pick up the code just to be on the safe side though.
with de-foulers, the ms3 seems to be a hit or miss. on other cars such as evo's and what not, de-foulers are a guaranteed way to avoid CELs but with the MS3's, people have been reporting CELs even with de-foulers slapped on. what CP-e does with their DP is they send you an o2 sensor extender. just moves the sensor further away so the readings wont be as concentrated so the lesser risk of throwing a CEL.
hope this helps