welcome! sounds like you have a great car and great plans for it!
interesting that you take a big hit in summer on your fuel mileage. i've had 2 different 4 cyl vehicles that yield better efficiency in summer even considering a/c usage. i would think most would have the same result.
my guess is that you either have mild winters or that 3 cyl engine REALLY struggles w/ a/c. i believe most of the members here that have them(3cyl vehicles--metro/firefly) live in the northern US or live where a/c is not a "necessity."
best of luck. i look forward to seeing you in the top 10! put your car in the garage so we can follow your results.