to answer your first question, learning black star definately helped me on my sweeping and speed. also eruption helped my on my finger endurance. crazy train was the first real song i learned all the way through, which i guess is always beneficial.
as for you second one. i live in chicago, so its cold a lot. no matter what temperature my fingers have to warm up, but in the cold it usually takes a little longer.
To improve technique and of course trying to keep all as clean as possible. I know my own limits and speed limits and so on I never play anything I'm not capable of. That wouldn't make any sense. After three years of playing I tried to play everything as fast as possible and that sounded, I would say, like shit, and I didn't realize that if I'd play bit slower things than I was capable of playing then everything would sound much better.
--Aleksi Laiho - Advice to Play By