Guess I'm the one swimming against the tide here.. In my experience, harder buckings show less difference between heat and cold, because they are... well, pretty hard to begin with. That's why I always go with harder buckings on my GBBRs, as the gas itself will quickly bring the bucking temperature down and cause a dramatic difference between the first shots and the following on soft buckings, that simply doesn't occur on harder ones.
Sure they'll provide less grip, but with the Autobot design that's not really an issue, unless you're trying to shoot ridiculously high polished bbs like Nov's or ceramic ball bearings.
So my advice would be to go in the complete opposite direction and get an Autobot 75 or 80Âş.
Maybe you can get both ends of the hardness spectrum and let us know how that turns out?