So for us non chemists....your saying BHO works better (higher quality and yield) at coldr temps.....like below freezing.....
Quality and yield rarely go hand-in-hand.
You'll get a higher yield in a lower potency product because all those undesirables add mass.
Higher quality at lower temps, probably higher yield at higher temps (in environment where pressure is allowed to rise). Water solubles being the biggest culprit as they somehow work their way into the non-polar butane.
Quick Chem Lesson, hope this helps: (I'm no chemist, so feel free to correct)
Water is known as a polar molecule (has magnetic 'pull'), and because of it's geometry/composition is quite polar, thus almost everything dissolves in water given enough time. Butane is non-polar, so that all those things that end up in a water extraction aren't as numerous with butane. Butane has a boiling point (where a liquid converts to gas, or vice versa) below your average ambient temps, so it is a gas unless under pressure. But is has a melting point (where the liquid turns to solid, or vv) well below that of water. So, if you can keep everything below the mp of water, 32, the butane will still be a liquid/gas and can move freely...However all the water solubles will be locked in the ice, because it's really hard to get something with 0% moisture in it, and any water molecules that may have contaminated your line/tank, will be frozen too...So it won't melt the ice, because the liquid is below the mp of the ice. I can see it hard to picture without some chemistry background, but water doesn't melt ice because it is a liquid, it melts it because it is warmer than the mp of water (ice).
Then is the challenge of purging slab at lower temps...The lower the temp the less it wants to leave it's new home.