So my OS X yosemite installer of 5gb took 25 minutes to download but updating to Devloper preview 2 (i don't know why it installed DP1...) took 4 hours.... and dev preview 3 of 1gb is taking 6 hours.
Will this mean faster downloads there?
Yes. Maybe.
This should help significantly when there are a lot of people trying to download something big around the same time: iOS updates, Mac OS updates.
But who knows if they will use it for everything... could be that developer downloads won't use this system for some reason.
They may also use it to scale up a heavy bandwidth service (like video/music streaming) to be able to handle more users and be more reliable.