Hey! I lost 20# by using my lunch money to build my car LOL!
The main reason that I would want to upgrade to rear disks is because they will be better for auto-X/road racing because there is less risk of fade from the disks, and if you would get brake fade on the rears, there is no telling which end of the car will be leading thru the next turn that I trail breaked thru...
That is however my opinion, not to mention , you can easily install the energy suspension bushings in the new ones without the car being out of comission (at least on a daily driver).