I was out the other day with a cold North wind on the bay. There was only a few white caps so I believe the wind was around 12 knts at most. I like to get the boat moving between 3.5 to 2.5 knts. I am trolling. Almost all summer there in these conditions I would use small jib and main. Yesterday with this setup I was doing almost 5. So I reefer and that only slowed me down to 4-4.5. Finally decided to drop main. Now I was sailing at 3-3.5 knts. Are cold winds heavier then warm winds making your boat sail faster. Also i was sailing across channel so current would be a very small factor.
To answer your question, air density at 80 degrees is 0.070 lbs./cubic ft. while at 30 deg. it's 0.081lbs. But since the major driving force was the Bernoulli effect, I think that the cold air gives a different perception, imho.