Y no superman?

so I question if he was gaining anything other than jacking up his danger profile).

Yes, he was gaining 13 seconds on Nairo Quintana. :)

I hesitate to lecture Froome on descending technique when Valverde, considered one of the best descenders there is, not only couldn't bring him back, but lost time.

That Pantani position is actually pretty stable and sae. I use it (or hands on top version) on about half my daily commutes to work. There is one hill where I test my speed at the bottom. I slide back on the seat until my belly button is right at the back of the saddle with my arms extended straight out in front. When competing against other cycle commuters, I invariably have 5-10kph better top speed at the bottom of the hill than they've got.

As I've mentioned elsewhere, my skiing background might be a help, but I think weight might not be too big a deal. I'm fat and 5'7" short. I have impromptu downhill races some mornings with a buddy. He's about 6'5" tall and a (now) chubby ex- pro rugby player. He's 20-30kg heavier than me, and even when he's doing all he can to go fast, I can still beat him handily.

I suggest that you try the Pantani position, in stages, under safe conditions until you get used to it. I think once you're comfortable with it, you'll be safe and fast. If you want to go fast uphill, we all acknowledge that you have to train for that. Likewise, if you want to go fast downhill, you need to train for that too. We all get caught up in this bike or that bike saves you 40 seconds on an IM bike split. Hell, I make up waaaaay more time than that, just by downhilling well and that's even on a fairly flatish course like IMNZ. On hillier courses I make up even more time on downhills. Too bad I've lost all that time already on the uphills :-(