Hi Tim,
I have a 2012 300 XC-W also and have the stock 13/50 gearing.
I just replaced it with the same this winter.
First gear is already low enough for me. Good for very very slow riding, but once I'm moving, I'm in 2nd or 3rd.
If you went with a 51 rear, I would think 1st would be unusable.
Unless you were trying to change the ratios in the upper gears, like 2 or 3rd.
Mark