RE: Great planes ultimate 160
How did you check your CG? Back in this thread there is a simple method. Make a mark on the fuse handle 1 inch forward of the rear bolt and balance there.That is what I use and it is good.
I use a DA-50 in mine but I had to move my battery far back into the fuse and add a little tail weight to get it to balance. I am sure the DA is heavier than the ST-2500, but not by 32oz, maybe 20 oz. But I would go with the 45cc, get better performance rather than add dead weight.