Is a pannier much heavier than a backpack? I wouldn't think the rack would make that much of a difference, especially on a commuter bike.

Sounds like a nice bag though. I'm currently using a timbuk2 which has some reflectiveness but isn't so comfortable once it's loaded up. I think my ideal solution would be something like the Ortliebs people have mentioned here that are meant to come with you off the bike.