I only notice decreased power with the A/C on when my engine temp is nearing 100, and I am accelerating from a stop. Otherwise, on the hwy, or around town, I don't notice a performance loss with the A/C on... I only notice decreased performance when I step on it from a stop with the A/C on and coolant temp close to 100d.

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)