The speck in serviceman's reference book is less than 2 15/16 stack thickness add one of each disc.The older book doesn't have a max thickness,but some here have stated it.When times where bad I have put a stack together with 14 of each,hoped and prayed until I knew it would still turn,which it did. Lasted until times where better and put all new plates in.
As for horse power I have operated 6 different D6 9u's,and there is a great variation in what you get out of them.What was my main tractor was turned up to 1750RPM or a little more.Now have one with a extra large dry air cleaner that may out work old NO 1. It came from a ranch that had a old Cat expert do all their repair work and he was known to help you get all there was in your Cat.
Cat ran the 318's in DW 10 scraper faster and with different parts in the pump than a D6 and claimed 120+ HP.Cat also offered a turbo in the other parts section of the parts book.Who knows what you could get combining the 2.