i've had 2 vectra's, the 2.0l sri and a 2.5v6 sport. the v6 has a bad flat spot between 2500 and 3500rpm, you'd need to fit a 3.8 bar fuel pressure regulator to get rid of this. the 2.0l was actually a lot better than the v6 and we got a lot more miles to a tank.

the v6 bhp is roughly 167bhp standard, then if you get the gsi version your looking at 192bhp.

0-60 is 8.0 secs for a standard v6 gsi, and 9 secs for a standard 2.0l, my 2.0l was doing a 0-60 in the same time as a gsi
hope that helps.