Will this release be able to run old 5xxx and 6xxx AMD cards along with 7xxx without performance penalties?
no
to clarify to the point of sillyness: BAMT 0.6 will not enable the use of multiple driver versions or multiple SDK versions simultaneously on the same rig. Therefore you will not be able to mix 5/6 series with 7 series in the same rig without performance penalties on the 5/6 series GPUS. It will however support the use of any one driver version and any one SDK version, including those that perform well with 5 and 6 series and those that work with 7 series. Deciding which driver and SDK version is best for your setup is left as an exercise for the reader. For people running 5/6 series only, there will be no change unless you decide to install 7xxx support.
BAMT 0.6 does away with the 'fixer' and moves everything into an apt repository. The repository has a main flavor and three optional flavors which are 5xxx, 7xxx and fpga. Selecting the 5xxx flavor will result in the same driver and sdk that we have always used. 7xxx will give you a newer driver which supports 7xxx but will reduce performance on other cards, as all drivers that support 7xxx do. The fpga flavor will make gold fall from the sky and submit bogus 1099 forms to the IRS in your name. The choice is yours.