Interesting posts. I'm very open minded about all of this and I like hearing your views.
Before the engine even runs the evo system is at a disadvantage in being an extra 16kg. I reckon that's roughly the weight of the spare wheel.
I think ideally you would mate the manifolds to a custom s/s mid section, for sub 250hp I don't see the need for the mid section to be twin pipe.. or the size bore that the m3 uses. Quite a bit of weight could be dropped off that.
The backbox can be gutted of all the sound deadening which would reduce the weight by quite a bit, this is something I am considering but I'd be worried about the resale value / the neighbors filing an antisocial behaviour order.
Having read up on the M3 manifold removal and refit, the rack link does need to come off which would have saved a lot of time in the first place if I'd started by doing that.