At times this route gets very close to 3000 meters (2965 to be exact) and even if it’s warm overall, it can get pretty chilly sitting outside a refuge watching the dying rays of the sun.
Assuming you can keep reasonably dry, keeping warm mainly comes down to keeping enough air trapped. The most efficient thing for trapping air without adding weight is down. I’ll be carrying a PHD Minimus with a Drishell outer (570 grams + 30 if you take the stuffsac).
I have been considering taking my MontBell Thermawrap (err on the larger size if you’re stuck between sizes if buying one) which has been great,when combined with a microfleece, sitting outside a tent at up to 700 meters in the UK this summer. But from my own experience of high altitude huts in the summer I know it can get surprisingly flipping cold overnight. So I think the 300 grams extra for the Minimus is justified.
Also from a safety point of view, turning an ankle in some of the areas of the HR could mean a few (hopefully a very few!) hours wait for rescue if it’s fogged in. Having something that could keep you really warm would be a very good thing.
I probably wouldn’t take it on something like the TMB but the Haute Route is a little more serious.
For some extra warmth from very little weight a microfleece is hard to beat. Chris Townsend wrote an excellent TGO article (“Lighten Up” May 07 TGO) that recommended the Jack Wolfskin Gecko. We’ve been trying them out and have been impressed by how warm they are for the weight. My large weighs 244 grams for my large and only 156 for LB’s size 10.
In August I’m not expecting to need gloves. However, a really small, light pair might make a big difference in comfort for only a few grams. We’ve both been very pleased with the silk liner gloves available from New Zealand Nature. Since they are quite cheap, only weigh 30 grams and are tiny I’ve bought three pairs and stuffed one into the pocket of the three warm jackets I use, so I’ve always got a pair handy.