The gain is about 10bhp, which on a road car isn't really noticeable, you may notice it revs slightly easier, it will accelerate better, due to exhaust gases flowing better due to manifolds, and downpipes.
Basically if you have a 3.2 just put 3.0L downpipes on,
Are you talking about 10bhp for the manifolds exchange only or for this whole list:
"3.2 inlet with injectors or fit 3.0 and the FPR from a 2.6/3.2 (slight increased rail pressure, worth having as it costs nothing)
3.2/3.0 inlet divider plate
2.5/2.6 heads which must be ported to match the inlet divider (2.5/2.6 heads have smaller ports than the 3.0/3.2) and 3.0/3.2 exhaust valves (sodium filled)
3.0 cams
3.2 bottom end
2.6/3.2 exhaust manifolds
2.5/3.0 front pipes and cats
3.0 electronics including DIS pack ecu etc with EGR blanked/removed and SAI removed"
If for the whole list then it's definitely not worth it. But have you done the mods yourself or are you guessing?
If really 10bhp only for all that, I think it would be just better to swap the engine for the GM High Feature V6 from the Vectra C or Saab. That gets you around 280bhp or even go up to 320 like the one in the Insignia, which are all based on the same engine as far as I'm concerned. Of course I have no idea what the trouble to swap that engine into an Omega would be. But at least we would be talking about real power gains.