Hi Chris,
Great looking new plane!!! Looks like you have the whole family in on it, that's great.
Hmm... 15% nitro with snow on the ground...wow... sounds like it's running a little cold, and that you might still be chasing microscopic tank shim that you otherwise would never notice when flying in "normal" warmer weather.
I would try a couple things... 1st, go down in nitro, but only go down incrementally, like mix some 10% and 15% to get 12.5%, etc, before you try straight 10%. I say this because a 2% shift in nitro is actually quite a large swing and you might hit magic in the smaller resolutions.
Also, 2nd, try an Enya #3, they are super hot, and the clearance seems about perfect for distance to the sweet spot, that has always worked a treat as my winter plug. Just make sure your glow driver fit's it and come's off easy.
As far as shimming the tank goes, it might be hard to quantify if it's close and only effects the run that way and in those conditions. You might have someone time the middle 4 inverted laps of your 6, and then time 4 upright laps after it's had 2 laps to settle down. Then time 3 inside loops versus 3 outsides. Even at that, it might be hard to quantify, but that's the best way I've found, other than just going by my best guess.
The .61 is a powerful beastie, and a fresh one with a good seal can be a little extra sensitive to ambient conditions and nitro...
EricV