I've been flying a kit built Goldberg Ultimate for about 5 yrs now. It is absolutely stock- don't reinforce the plane, it is just fine as is.
I did put dual aileron servos in the bottom wing instead of the bellcrank.

I have put mine through the mill, never broke anything. Well, did have the elevator hinges on one side come out this past weekend doing knife edge spins- those get pretty violent.

Mine weighed 9.5 pounds when I had the Saito 1.50 in it. Very overpowered, landed "heavy", but never broke anything. Tried a Saito .91- flew a lot "lighter" great multi-turn flat spins, etc. Just a little shy on the vertical. Weighed 8.5 pounds with .91.

Tried MVVS .77- excellent power- like the Saito 1.5 but light like the .91. Too many deadsticks though.....

Now using Saito 1.00 which has to be about the best 4 stroke for it. Power like a 1.20, weight like a .91. Consumes fuel like a 1.20.

What would I change? The upper wing mounting is a bad design and hard to assemble at the field. Trash the single wire and build mounts like the DP Ultimate. Might consider a lighter landing gear.

Also, I didn't like the bottom wing attachment with allen bolts- in fact mine tended to become loose and drop out after a flight. Converted to 10-32 nylon- no more problems.

I have read about a couple of GB Ultimate's that weighed 7 pounds. Have no idea how they made it that light.

Ken