dont know much about 400 series cars, on my old honda 100hp the ac really bogged down the motor, so may just be you dont have that much hp and a 20 hp loss is noticeable. Have a mechanic check the compressor to see if it has excessive load, i dont think overfilling the system wth freon or 134a would do that though. If its overfilled a lot (like you added more than a 12oz can of freon) then id have it evac'd quick.