cav or vectra non DMF flywheel won't fit in Omega, either.
the friction face is further away from the rear face of the engine on Omega to give room for the dual-mass gubbins. So with the "transverse" flywheel, you'd run out of release bearing travel before releasing the clutch. you could, of course, space out the release bearing assembly, but that sort of thing isn't exactly a straight swap.
the Cav or Vectra flywheel isn't going to be that much lighter than the Omega one, and if you want it to be "livelier", then you're really after a flywheel with a lower rotational inertia. That's aftermarket territory.
I dislike the DMF, too - it's fine for comfort and low NVH, but not really any good for more sporting ventures. I had a replacement aluminium flywheel made for the Omega Evo.
Just for reference, the DMF applies both weights to the engine when you're out of gear (smooth idle), whilst applying only the inner weight to the engine when in gear and driving - the outer weight is applied to the transmission to dampen drivetrain vibrations.