I'd keep the cold side piping the same size as the turbo outlet.
The FFP seems to have varying results, I wouldn't put one on a car now but also wouldn't take one off if it's already on there.
If e85 is not convenient could do the e42 mix.
There's a post from Status here somewhere saying the 8cm doesn't seem to be that much more responsive but loses about 30 or 40kw up top so if doing something with the turbo I'd probably go to a BW or PT.
Stock cams & TB may make it feel a bit better in the mid range.
E42, more boost & maybe head studs