Sort of makes you wonder why GM didn't pick and choose a bit more and transplant the Saab unit into an Omega - or even a Vectra. I mean the ST / GSi 'only' had the G-cam'd X25XE with 195bhp - as you say, with a remap of the Saab Turbo you get 258bhp, all from a 2.3. Perhaps in the case of the Vectra Supertouring/ST200 there was the Homologation element to it, but not such with the GSi, or any other 'sporty' version they may wished to have made.
Ok, perhaps the Vectra a bad idea with chassis limitations, plus at this time late 90s-ish there hadn't been a hot hatch over 200bhp (
I think - wasn't there the Alfa 147 about then, allegedly you 'couldn't' but any more than 200 brake through the front wheels? Wasn't until Astra VXRs came along that Ford/VW etc started doing 200bhp + hot hatches?) Ok, defeating my own argument with the Vectra perhaps, but an Omega Turbo would have been a nice option. And considering the 'cheap' engineering involved. (in car Design terms cheap, anyway, engine sharing across marques very much commonplace) makes you wonder...
