Be good to hear the results from the latest shoe/boot trials.
I am sticking with a cheap but light tent, done a lot of camping and I think a good night sleep will improve your time better than most kit upgrades. I agree, I don't think a bivvi alone with give you enough mental comfort to sleep well.
Interesting about footwear, I was ready to buy some speedcross 3. I have been training in some la sportiva trainers that have good grip (only slipped twice over last few weeks and that was on wood bridges). They do suffer from lack of cushioning on rocky downhills and have no waterproof abilities. I bought them a year or so a go wanting the best grip I could find but only because I had broke my leg previously slipping on a grassy bank. Hmmmm what to choose....
Wow, got an idea of what the event is about. Took my step son and went to walk across Bleaklow last night. Pitch black, no light not even a star to glow. Thick fog and visibility down to about 3 metres. Made nav very interesting but just about managable with map, compass and GPS but very difficult. On pennine way could cover reasonable pace, once paths were vague, slowed right down., really right down. We went across the moor and by midnight decided enough and pitched the tents. Can't imagine how eiry this will be alone and tired during the race. Coming back we could check the route this morning and some very difficult sections were just difficult because we couldn't see the right path a mere metre away.