Thanks for the reply. In any case, I risked it and went for a ride with the post 1 cm over-extended. And by Zeus, what an ride it was!
Uphill by road I was a bit slower as expected, so no surprises there.
But once I got onto the trails, I rode noticeably faster. On the downhils I smashed all my PRs of course. But what really surprised me is that on all, except one, uphill section I was also faster than I have ever been.
Was this down to better traction? Yes, or at least partially. I found myself able to clear tricky steep, loose switchbacks that I usually have trouble with. I also felt fresher coming out of the downhills, so perhaps the suspension also saved me some energy that would otherwise get eaten up by trail buzz and bigger impacts. Then there's also the potential adrenaline factor - riding a new bike and the extra fun factor on the downhills might also have led to a boost in adrenaline levels, making it a bit easier to put in a harder effort on the uphills.
That one section where I was slower is half green lane consisting of bedrock, and half gravel fire road. The green lane section I have only ever cleared in one try while riding rigid. Suspension and higher / wider bars just seems to make it hard get the required steering precision there.
As always, there are confounding variables which could have affected my times. Since June or before, we've had virtually zero rain and the trails were extremel dusty - then at the start of this week we've had torrential rain non sto for a few days, which seems to have dampened down the dust and changed the texture of some trails.
Time will tell, of course. But this is looking to be an excellent bike, and may even motivate the purchase of a modern 29er XC suspension bike in the spring. Maybe...