Re: C&C 30MK1 main sail slug size?
Excessive heel will slow you down and increase weather helm, which will also slow you down because you'll be applying constant rudder to hold course. There are lots of things you can do other than just easing the sails off. I'd highly recommend a couple of books: Sail and Rig Tuning, Ivan Dedekan or North Sails "Fast Course". They'll tell you a lot about adjusting your rig and sails for different conditions.
The C&C 30 will handle quite a lot of weather if you know how to tune it. Ease the traveler then the mainsheet if necessary to reduce weather helm. Flattening the main (tighten outhaul and vang) will help depower. tightening the backstay will help flatten your genoa. You'll want to reef the main or downsize the jib if you still need to depower. During a race upwind if I'm overpowered I often just let the main off until it's backwinded considerably-- C&C sailors jokingly refer to this as a "speed bubble."
There's really a lot to learn and I'd recommend the above books
1977 C&C 30 Mk 1 hailing from Port Clinton, Ohio