That absolutly normal. A/C take about 10-20% of your power. A/C needs a lot of power to make the compressor running. And you'll do less MPG with A/C on. It's like driving with all windows open. The restriction caused by wind with open windows reduce aerodynamic coefficient resulting in less MPG too.
yeah it happen with AC. It's worse with a 4 banger. Sometime I just shut off the AC momentairly when I need to get on the on ramp to merge into traffic.
but i hope for you you don't have a very bid sound system because underdrive pulley like AEM Tru-Power reduce amp and watt.. and anyways, there is no real hp gain of using Tru-Power pulley..