Bilstien B4 or B6's work well with the Omega.
Steinmetz or Eibach do a spring set which again work very well on these cars.
Anything lower then 30mm from standard will cause problems when you setup the suspension due to the suspension design and pick up points.
Imho, Omegas was never designed for coilovers so personally I wouldn't go down that route regardless of what bullshit the kit manufactures come out with.
If you really need the harder ride that 40mm springs will bring, you can fit (if you can get your hands on some) a extra set of the rubber pads that the springs sit on thus bring the car inside the 30mm realistic limit

As ferret features says, remember to blank off the self levelling pipes / pull the pump fuse regardless of what you fit.
Just remember that the Omega weighs in around 2 ton with a lpg tank.
Its never going to handle like a sports car but if setup properly, you will be pleasantly surprised just how much you can throw these cars around and not end up in a ditch / or with a knackered back
Have fun
