If you lower the car, you'll HAVE to fit a camber adjusting kit. Otherwise you'll be trashing new tyres in less than 1000 miles. Regal Auto do a kit for about £200 (I think that's what I paid, but it was about 3 years ago !).
You'll then need to have the steering geometry adusted.
The four wheel alignment that you get at "Kwik Fleece" etc is not what you need if lowering a car. You'll need to contact your nearest Performance Car/Rally preparation centre.
Also if you're gonna run a 45 profile you'll need 17" rims otherwise your rolling radius is gonna be wrong. You should be running 15" already so 16" are neither here nor there.
You could fit 19" with no arch modifications on the Omega... but the cost of the rubber would be as much as for the rims and it's just not practical on u.k roads.
I'm running 235/45-17 with 40mm lowered springs and new (but std) Monroe Shox. I reckon 30mm lowered would be better, as I've had to junk the under engine splash guard (Damn speed humps !!).
Mind you, just a new set of 17" rims with the 45 profile rubber will make a big difference as there is significantly less flex in the tyre wall than the standard "mini bus" tyres.