I recently built up a 2013 26" Five. The aim was to produce a bike that was fun and gave a bit more confidence going down, both of which it delivers in spades. I expected to give up a fair bit in climbing and general XC ability, but I've been surprised at how little it really loses. OK, it's a new bike so we have to factor in the honeymoon effect, but it's been a bit frustrating really. I have a 29er HT (FF29), which should kill the Five for speed, but doesn't, which makes it hard to justify. The FF29 is faster, for example, on a long not too technical climb. But even there the difference seems to be less than 2%, which might matter if you are chasing a podium spot, but doesn't amount to much for the rest of us.
Being a single pivot the Five does rely on the shock and you do need to fiddle with the lever if you care about speed, but fortunately it's right there in front of you.
As for making the Five shorter and steeper; I doubt you'd gain much, if anything. My previous bike was a Trance, which is a fair bit steeper and shorter travel than the Five, but I haven't seen any difference in XC speed between the two. In fact, at the moment, I'm beating all my previous records on the Trance up and down, but again, that could just be a honeymoon effect with the new bike.