Have been running MMs on B courses and upwards for last 13 years in all sorts of conditions - started out with a horrendous kit load and have made plenty of mistakes over the years, but now running light is making a big difference ie top ten finishes on long score pretty consistently now.
On balance I would stay away from down except for sleeping bags, which I always double bag separately to all other kit including in at least one canoeing-type dry bag. I agree Rab photon jacket brilliant, especially when you get in wet with sweat at the midcamp, and need to throw something warm on quickly perhaps in the rain even before you have put your tent up, often over your sweaty baselayer. I always take a buff or two as they are great to run in if cold but not too hot, and can double up as a scarf at night, help midge-proofing [always take a head net!] and can be used as a headover when sleeping as well as a hat to up the rating level of your sleeping bag. I take a very light sleeping bag and don' bother generally with a thermarest or similar - bubble wrap works fine, especially large bubble type doubled over on itself. You don't need anything under your feet, just your torso. I run in lycra shortsand sleep in full length [ 2XU] compression tights on top of these to reduce soreness and overnight cramping. They are very light for 'full length leg covering'. Waterproofs I use old paclite 3 stuff top and bottom from Berghaus, and take a Montane featherlite windproof to run in if cold / up the weather proofing of the jacket if really bad conditions .... [KIMM Cheviot Day 2 anyone ...?]. Make sure you measure out all food so you finish with the regulation minimum left and nothing more - I always seem to end up with a few bars and gels over no matter how hard I try to eat. We always take 2 main meals for the evening to really aid recovery - we always have a better day 2 than day 1 relatively speaking so it seems to help.
I never drink the booze if available at midcamp - had a bad experience after 'only a couple' at the Highlander a couple of years back - slept cold as a result and woke up at 3am and -9C and couldn't get back to sleep thereafter.
Best tip I have though re kit and weight is to lose the belly - dropped 4kgs last season I raced kind of accidentally and really noticed the difference. Not much point binning the toothbrush from the backpack if the family size cow pie is still attached to the front of you....!
Hope this helps, have fun....