Originally Posted by
astage
Thanks. I'll look at that once I've decided whether ornot to keep the original saddle - still trying to accustom my rear end to the pleasure!? 
I managed to swap the stems over without much difficulty and then turned the seatpost around - which brought the saddle's pivot point more in-line with the post's centre line, even when pushed back. So far I've had no slippage even with yesterday's deep pot-hole.
Based upon a couple of rides the new saddle position is much more ideal, with far less effort needed to peddle because I'm no longer peddling from behind, and I'm in at least a gear higher (and faster) on those inclines. Not yet tried to ride out-of-the-saddle, butthink it should now be possible.
I still have to fine adjust the handlebar height and lever rotations to lessen palm pressure, but looking better and enjoying it more. 
Make sure you use loctite on the pinch bolts, the only weak link to the Birdy is where the fork bolts to the steering tube.
I converted my Birdy to an XTR 11 speed drive line. It was a bolt-on conversion, and the wider range of gearing allows for acceleration and climbing without having to get out of the saddle. A firmer rear elastomer, spring, or shock makes out-of-saddle pedalling less bouncy.
Get yourself fully broken-in before making too many adjustments. Once your backside is used to the saddle, you can then make adjustments to increase comfort, deal with numbness, etc.