I'm no A-main driver, but I've switched to the 103 front arms on my 104 and there hasn't been a transformational change in the handling. Perhaps there are some other issues lurking in my setup, but I'd rank tires as the biggest single influence over how the car goes.
The weight distribution of the battery between the two cars is most likely also be a contributer to their relative performance. Putting some weight over the back wheels helping is possibly a testament to this.
Either way, I still have a blast driving it!
Ran close to a 70mm rollout at a large outdoor track recently with a GTB hooked up to a 17.5T and it was simply good fun. Motor was only starting to get above ambient temperatures, so I probably could have gone harder.
So 103 arms with 103 tires front and rear? 104 rears make the car much narrower.
As far as my comparison, the thing to me is that the 104 probably has better tires at least with the foams. The rim is larger, and the compound a bit more suited to carpet, but the geometry I think holds it back