I have three, shoot them all the time. Having the gun cooler than your CO2 tank definitely helps in getting a good fill. Sometimes, I remove the gun from the stock and shove it in the fridge for 20 minutes or so before filling. Other times, I'll immerse my 20 ounce paintball tank in a bowl of warm water for a bit. You don't want it to warm, of course. My two favorite 114s are crazy accurate. I have put stronger hammer springs and lighter trigger springs in them, both wear vintage Weaver scopes, they are great shooters.
Roll pins are supposed to fit tight, if they didn't they would come out to easily. I hate whacking on them but it's the nature of the beast. I found an assortment box at Harbor Freight pretty cheap, they are black.
To answer your question, a 25 degree difference between tank and gun will probably result in a better fill than 15 degrees. Heck, I probably go for a 30 or 40 degree difference. I have also noticed it's easier to get a good fill when your CO2 tank is closer to full.