I have not found a significant increase in velocity from changing nipples. As for igniting powder better in cold weather, There IS NO PROBLEM igniting BLACK POWDER in Cold Weather.
I cannot say the same for the Subs. The Ignition temperature on the subs is Higher than for Black Powder. In extremely cold weather, that may make it a problem. However, If you have cleaned the flashchannel, and Powder Chamber or breech end of the barrel with alcohol, to remove ALL grease and oils, so that the powder is able to get back down close to the nipple, you should have nothing to worry about using any nipple to ignite the powder.
You may have to use " magnum " #11 caps to ignite the subs, however, rather than standard #11 caps, from what I have been reading, and hearing, from shooters who are using the subs.
There are other valid reasons for spending the money on those more expensive nipples. The Uncle Mike Hotshot nipple has a hole located in the forward portion that vents gas to prevent the hammer from being blown back enough to allow the spent cap to go airborn and pose a danger to the shooters, and by-standers. The gas instead exits under the side of the cap, blowing it forward, and making for Easy Removal from the nipple, when the hammer is moved to the half-cock notch.
The Beryllium-Copper " Spit Fire" alloy nipple is made from extremely hard, tough metals, that resist being deformed by the usual hammer falls. The metal also allows the Orifice inside to keep its size, instead of being slowly eroded open as happens with softer steel nipples. The stepped design of the orifice is claimed to improve ignition and increase chamber pressure, thereby increasing velocity, but I have see no testing results, other than the stuff put out by the factory, to verify that claim.
As a general principle, the smaller the orifice, the less gas can escape out through the nipple, and reduce the velocity of the ball or bullet going out the muzzle.
However, there are trade-offs, and not the least concern is having an orifice that is large enough to be cleaned out IN THE FIELD, with a standard wire found on most BP tools. Nipples( and flash channels) get plugged, on occasion, from debris from the percussion or musket caps, from residue of black powder, AND THE SUBS, oil, grease, and even fibers left from cleaning patches that don't completely burn up in the barrel.