Volvo Repair: 1994 940 Turbo A/C & Power lack, excessive load, sw florida

Question
Dear Regis,  I live in Miami where A/C is a must. My 1994  Turbo 940 used to bo a 'rocket' with a great working A/C until the day my mechanic broke the original Volvo compressor and replaced it with a cheap aftermarket piece. When the A/C is turned is makes a little howling noise and the cars power is reduced by a good 20%-30% or so. (I know that a running A/C reduces the power of a car but this is far too much). The original Volvo compressor did not do this. Furthermore the car does not always shift down when accelerating.  Once the A/C is turned off, it's down shifting fine.  The A/C is also not cooling good, unless driving at 2000 RPM or higher.
What could the A/C compressor have to do with the shifting ?
I appreciate an input of yours - thanks !  

Answer
the new compressor or the rebuild which it may be, might be bad. I have not heard of this before but if it started after the compressor replacement then it could be the compressor.I would have him check it out.the compressor could be causing an excessive load on the engine when it runs.A/C is also a must here in SW Florida.