I always turn it off, just in case. The only problem would be that you're stressing the compressor clutch (automatically disengages at high rpms to prevent damage), but if he does it every single time, it's probably the same thing.

And, yes, it does make a difference. The guys at Speedlab showed us a B16A Civic on a roller dyno, being held at half-throttle... turning on the blower and compressor together cut maybe 15 hp from the engine's power. Yes, the SiR has a small engine. The only time the compressor will make a negligible difference in performance is on a big block or a 300hp + turbocharged engine... for us small N/A guys, it's like driving through molasses.