Now now, boys and girls (or is that boyz and girlz?), play nicely...
To answer the question, if the manufacturer claims they are for an Omega, then they should fit standard models. If not, assuming you buy from a bona fide company, you have SoG to get a refund (but no refund for fitting work and geo work). Not sure many people here will know specifically about that brand of lowering spring for the reasons below.
As chrisgixer points out, the Omega is designed around its standard setup. Its actually very capable in corners in the right hands (and if anybody's isn't, then either the suspension is tired, or its in the wrong hands). Its certainly possible to utterly destroy the handling by overlowering, in the same way you can bugger up handling by over tyre-ing a car.
Having spoken to people who are trained in car design, I vaguely understand some of the points and reasons for some of the components, and why they are designed as they are. Most things in the suspension are going to be a compromise, and the design of it puts markers as to how far you can stretch that compromise in another direction. The biggest advantage with RWD, is you don't have to compromise towards bone shaker suspension just to get the power down, as is the case with any powerful FWD car. So you only need to compromise between ride comfort and cornering ability.
As you lower, you need to stiffen to reduce any bottoming out, and this reduces the car's ability to soak the bumps, which tends to result in the car getting a bit skittish in the corners, as the tyres fight for sufficient grip over any undulations.
So, my own personal opinion, if the manufacturer claims they will fit, who are we to argue, but I do think its too low (as in too much of a difference to the stock design), and will make a mess of the handling (though those coming from FWD might get the placebo effect, believing stiffer must be better). You will also be chewing through tyres at a decent rate of knots, so you will need to budget that in as well.
This is why I, personally, wouldn't recommend a drop of more than 30mm as an absolute maximum from standard. Irmscher/Steinmetz springs with B4 shocks seem to be the preferred setup for those members lowering their cars, but wishing to retain good cornering and most of the ride comfort.