I have a Vectra 2.2 Auto (53/C model - pre facelift) and it goes like a rocket. 8.8 to 62 (rolling road test) 158bhp. Doesn't look like there are any mods to anything other than the shocks and there is a Pipercross air filter. I got it second hand like this. Not bad for a car that has nearly 89k on the clock. I think that maybe the gearbox has been tuned, but I have no way of telling.
I would stick with a better air filter and maybe get uprated shocks and springs at a later time. Mine is the elegance model and doesn't seem to be lowered but it holds the road better than my old modified MG-ZS which is also excellent.
Don't go mad modifying alloys. Standard 17" are great but bigger increases mass, reduces performance. Unless you go for super light versions.
A better air filter is a cheap option and may improve MPG too. I get 28.5 average, 48 on motorways.