You should hit 400 with custom exhaust, performance filter, larger FMIC and a good map. Or close to anyway.
The recall thing has been discussed lots. Vauxhall chose to bury their heads in the sand, then charge customers £3k+ for a new diff. And if unlucky enough new propshaft and gearbox as well.
Stage 1 will probably add 20-30 hp, but around 100 mid range. It opens up the mid range more rather than top end. It is a huge difference. With a decat, I'm sure it was a stage 2 map.