I have a rab alpine and have had good fun with it, only problem was the silnylon on the base wore out, and i cannot find any seal tape to stick to silnylon, i have been told there is one that does.

That is an important point as i found when i made my own bivy, make sure your tape (if you use tape) sticks properly, pvc i think is the norm. Sone material as i found out gives the impression of sticking until you fold it and use the bag, then it peels away, and your left miles in the sticks lying in a puddle !

The 3 layer goretex is good, fully sealed i still got condensation, the down bag eventually rotted around the face leading to less loft, if i wanted to breath into the bivy i would not make it from anything else, but if your breathable material is any good (and the budget stuff has come along way in 20 years) it should suffice for all other