IMO, there is absolutely no way that the ZF is Extra Stiff - it'd be the same as the NS9000X if it were, and it isn't. The last true Extra Stiff Yonex did was, for me... the 9000X. The PPro and Rev aren't that stiff, but they're certainly stiffer than, say, an NS7000 or Arc 10. I thought the UP was brute force incarnate - all power and foritude, but no subtlety or poise. I think it was at least 25% more powerful than I could get out of it. The Rev has comparable strength, but more accessible power and far superior feel and control.