Mercedes Benz states " compressor boost can be reduced up to 40% at the shift point in order to protect the gear box. Normal, do not replace any parts". That's a direct quote.
I don't know what variables the vehicle needs in order to reduce boost, but I've never experienced it. I would imagine it senses the difference in speed sensors inside the trans. I've beat the crap out of several SL55's and never felt like the compressor was being cut back. Good luck.