I've got a slim straight through back box, replacing the original large box. There's a slight increase in noise, but exhaust from turbo vehicles is lower, simply because the turbo absorbs lots of energy from the sound.
That can be done on the pre 2007 FL2, D3 and RRS, but not on the FL1. The FL1 used a now old fashioned flasher unit, unlike modern vehicles that have a BCM for such tasks.
So the FL1 needs an add on box of tricks to do the 3 flashes. I couldn't do without it when I got my FL1 so found a way to add them myself.