The 511 comes with cheap nasty ball studs that shear off before you've actually tightened them up
I wasn't impressed with what you got for your money with the 511 - the 501 with worlds-sized belts and pulleys was and still is an awesome car.
As has been said - other than changes to make working on the car 'different' - the main difference was the cells being 6mm further forward, but that's no longer the case with lipo's.