No, the battery won't be too heavy. I haven't made the switch to LiPo yet, and all my packs on my GWS planes (Estarter, Formosa, Slow Stick) are just one cell smaller than yours. The extra .7 ounce would make a bit of a difference.

I think the 400 SS comes in a direct drive version. I wouldn't recommend that one. Get a geared drive version.

The 400 is about an ounce heavier than the 300. Your heavy duty ESC also adds a bit of weight. The SS is pretty good at packing a little extra weight, but keep your expectations within limits.

The 300 would save you an ounce of weight, but you'll find that you'll get a bit more lifespan out of a 400. Just thought I'd let you know, in case it matters. You can't have it all, unfortunately.

I would expect your flight times with that battery to be around 12-15 minutes long, depending on how you fly it. Full throttle, you might get 6 to 8 minutes.