Decoding does not change the total signal (although I think this might be based on keeping something else constant like RMS so you may see some changes in the final dB levels.)
Decoding raw m/s results in some summing in the stereo mixdown, up to +3dB or so from the relative values of the raw sources in my experience with Digital Performer/AudioDesk - and it seems that should hold in general.
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Yeah, I have corrected it. According to the CEP help channel mixing is not additive, but I usually see a couple of dB or so increase as well. I've never liked the approach CEP uses to M/S as opposed to the usual "ratio" concept, but I don't run it enough to really work it all out or need a better DAW.