You'll always get a power drop with air con on. Though it shouldn't really be noticable on a 200+ BHP car as it only takes a a few BHP to run.
I drove my car for 50k miles usual blend of hard driving, cruising and pottering around (same as most people) and the aircon was never off......yes even in winter.....never had any aircon problems.
I've been on these boards a while and aircon problems are NOT common on the LCR, the odd recharge needed by people etc but by no means common on the hundreds we have had on the forums.
In winter the air conditioning cuts down the amount of moisture in the car I found (the conditioning bit of the aircon) and therefore less time de-misting etc.
I could never tell the power difference with air con on or off besdies a remap takes care of that issue