Those KAN950s are pretty nice cells. I had no doubts they'd power a Speed 400 just as well, if not better than, the 1100 NiCds.

Bigger motors are not always the solution, as you've found out.

Did you use the standard radio gear? You could easily save about 3oz there by going to micro servos and a smaller receiver. Another thing you can do is go to a 9-cell pack, but don't expect long motor life.

You could also look into a Lithium Polymer solution. This would really lighten the plane up.

I also remember reading about someone who used a geared Speed 300 and 720mAh AAA NiMH cells. The plane weighed about 16oz, IIRC, and flew quite well.