I didn't think the 350c motors needed to be retimed. I only retime the old GWS 300 motors - and only about 3-4mm at most - line the red dot up with the crimp on the casing.

I use 3s 1200mah lipos with my GWS 400 esc with no problems. 12 oz plane hovers at a little over 1/2 throttle with the old GWS 300 C gear. I've used both 11x8 and 10x8 prop.

Maybe try a little higher pitched prop or a 3s lipo.