RE: Scratch build speed plane.
A average quickie with a normal 25 will crack 90 with little effort. A clean well designed one will hit 100 with a sport .25 engine. With a .46 you should be able to do around 120-125 with no special effort.
It is not necessary to try to save a lot of weigh if you are only interested in straight line speed. Are you planning on covering your carbon tube truss or what? I don't think that it will buy you anything over a normal balsa/ply structure.
1/32 balsa will not be enough by itself to hold a wing together. The top skin will fail in compression. Normal skin thickness for something slow is 1/16", and I use 3/32" or even 1/8" on the top skin for 160+ mph quickies and 1/16" with 1/2 wide carbon on the bottom skin.
G loading is determined by the speed of the model and the radius of the turn. Racing airplanes have to be able to withstand loads in the 30-40 g range. It's always fun to see who did their homework and who didn't. Racing with wind gusts really stress the center section. I went to a race in Texas once where 20% of the wings failed due to the conditions.
If your only interest is going fast on a straight line, I would also consider cutting down the wing span by 6-10" if using a standard quickie wing airfoil. Or better yet, copy something like a Q40 for a bit more speed. You can buy some Q40's with foam cores for low money and go a lot faster.