Your 50 AMP ESC is way more than you'll ever need, so you're good to go there. I draw, on average, about 3 amps out of my pack when flying my SS. Full tilt, you're drawing around 8 AMPs. (You want the rating of the ESC to be bigger than the amount of amps the plane will draw. The ESC does not determine how much current flows to the motor, it just needs to be able to handle the amount of current that will flow to the motor.)

What motor do you have on your Slow Stick? Is it the 300 or the 400?

That battery would be great with the 400. It is above the recommended voltage of the 300, but if you use it with a 11x4.7 prop (GWS calls it a 1147, I think) instead of the stock prop, you should be good to go.

I take it you're well aware of the way this aircraft handles the wind?