Not had it on a r/r yet, plan to before long now that my FRP conversion is complete. It should make 155BHP, but from what I understand many FRP's make less than that. I would be happy with anything much over 140BHP.
Is a 4-2-1 worth it? Probably, so long as you are aware of the difficulties with your check engine light on later cars, and that it will be noisier. Certainly you are doing it cheaper than throwing £1k at a full Milltek system with sport cat. I am sure the 4-2-1 does increase power as the standard close coupled cat arrangement is quite restrictive, but it wasn't as significant as I had expected.
If I was to do it again, I would probably start with cams and a remap, just because the noise of the 4-2-1 is significant, especially on the motorway. Having said that the noise is quite pleasing when you are pootling around town, and especially so in multi-storey car parks.