25g or so per extra 20mm for the disk, and roughly the same for the mounting kit - so 50g per wheel per extra 20mm.
If your brakes are not feeling short of power and your not cooking them all the time on scary alpine descents I doubt you need bigger rotors.
205's are very prone to rock strikes so if you can get away with smaller I would.
Question I've been wondering recently is wether to go for a lightweight brake package with a big disk, or go for a more powerful brake with a smaller disk -the brake systems would work out at similar weights and it would be interesting to find out if there are issues with either option.
Hope X2 with 203 vs Hope M4 with 160
Avid Elixir with 203 vs Avid Code (2011) with 160.
Interesting experiment for a mag to do I think - I think I'll make this question a thread of it's own...