Never ran RetroPie on the B/B+ but did run Carles Oriol’s ChameleonPi which is essentially a remix.
RetroPie on the Pi 2 is great – full speed PSX with no over clocking.
Plus the Pi 2 has enough grunt to cope with the deficiencies in BBC Micro emulation so you no longer have to run RiscOS just to get an ARM-optimised 6502 emulator that will do it.
I wrote a little blog post about it. Maybe somebody would like to include that machine , despite it being a bit UK-specific. Build instructions at: