Yes on all accounts of your questioning. Quality of barrel, length, care of said barrel, and metallurgy will cause barrels to shoot differently. Most ballistics for a 7mmRM are based on a 24" barrel. If you are shooting a 26" to 28"' barrel you can expect a gain of 100 to 200 feet per second. If you have done a good job of taking care of your barrel, then you will be able to see some velocity gains as well as accuracy. Sounds like a good set up. I am determined to buy a Mauser action some day and do a classic Mauser build. Germans definitely knew what they were doing.
Temp, altitude and humidity all factor into a bullets performance. The warmer and more humid the weather the better a bullet will fly. I'm not sure why, I can remember reading it some where that humidity helps a bullet fly better. If you play with humidity setting you will see this change in your ballistics. I could be backwards, it has been a while since I have read that article.
Tank