If you can, replace as many 1/4-20 bolts with smaller bolts as possible. 228 used to use 1/4-20 for everything, since they were easy to pick up at Home Depot. Then I calculated that by dropping to #10/32 for almost everything, we could save almost 3 pounds; now we exclusively use #10/32 button and socket cap head screws.
Lighten the gears in the Toughbox. Turn pockets into them on a lathe, and/or drill six 7/16" or 1/2" holes through them. This can cut their weight in half.
Don't use 4 CIM motors on a drive train. Use 2 CIM motors, and use chain to connect the other wheels. CIM motors weigh 2.75 lbs each.
Take a second look at material choices. You can do a lot with 1/16"-wall aluminum box tubing and 1/16" polycarbonate (Lexan) without breaking the weight budget.
Using pneumatics? Only use one or two accumulators. They weigh a pound each.
Edit: But most importantly, treat this as a weight-per-effort
knapsack problem. That is, go after the densest materials first that take the easiest amount of effort to remove a relatively high amount of weight. Then keep working downwards towards the parts where it'll take a lot of effort to only remove a little weight.