Funnily enough, last few times I've been there I've gone prepared for the cold and ended up having to strip off. One of the useful things about Springwater is that even if the day is windier than you'd like, you can still find some bank where the wind works to your advantage.
Last time i was there was at the end of february. Dead calm for most of the day and sunny. A hare's ear nymph on the top dropper off a floating line with a long leader, fished dead slow did some damage. A viva lure (black/fluorescent green) fished quicker on an intermediate line took a couple over the shallower areas. In the past I've had them in the winter on buzzers too so I spent a good while fishing deep into the deep water to the front/left of the cabin but didn't get a touch doing that.
Your best bet is to ask in the lodge when you buy your ticket, check out their website, and check out Wullie's update posts. In a couple of weeks the fly life could be a bit more active.
springwater ayr open
Oh, and I think the website says they open at 8 but when I was there they opened at 9 - worth getting in touch with them to check if you are planning on getting there at the crack of dawn.