Buddy,
The newer autos than my 05 auto are better yet. and the 05 allison is way better than my 2000 4l80e, and milleniums ahead of a th400 that I had behind BB gas rigs back into the 80s.
I would have no issues with a 6l90E behind one of GM's 6.0 gas V8's towing my 12K equipment trailer on roads way the he!! steeper than you are talking about, ie 15-20% with some in the mid 20% range. 8-10% are nothing grades in my book. i used to live on a hill that was 9 going one way, 11 the other. I know of many roads in downtown Seattle that are in the 20+ range......like I say, 8-10 are shoulder shrugger jaunts up and down.
marty