/ Ridge Raider as summer alpine basecamp tent
You could take a fairly roomy tent for the same weight.
I think your life will be miserable in a bivvy for that long. I would buy something like this and hump the extra kilo http://www.argos.co.uk/static/Product/partNumber/9026542.htm
for £27, give the bags and mats to a street sleeper and you have a tent that weighs 2.2kg with room to sit up and read during those long rainy evenings.
15 days isn't as bad as 25 and it depends where else you have available to go, for cooking, when it's raining etc.
But I'd think about a tarp to offer a bit more protection, storage space, somewhere to sit when it's raining, cooking, etc.
example here. http://www.flickr.com/photos/brek/8548646104/in/photostream/
Not sure I'd like a multiday trip like this though.
I'd take either a Laser Comp or a Power Lizard. Bigger footprint than a bivi mind, but same weight or less than the Rab bivi.
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