The V6, especially the 3.0 isn't really what I'd call smooth, especially not when you nail it. And the economy is really quite poor when measured against a BMW 4.4 V8 of the same era that can do 30mpg. And it really isn't fast by modern standards at all. It would easily get battered by something as mundane as a C220 CDI from a standing start. Gearing doesn't suit (slightly illegal) cruising speeds either.
Not to say I don't like it as an engine I must stress.
A V6 by pure definition is smooth, no exception with the X25XE, Y26SE, X30XE, Y32SE. A properly running Vx V6 is like silk.
Same era V8 BMW stands no chance in the 'economy' department. My e39 535i would struggle to get above mid teens, and the 339 is lighter than the Omega by a fair amount.... step up to the 7 series, and it's laughable. Ask me how I know!
Comparing a 20 year old car to a modern diesel is like comparing a Mother to her Daughter - Miles tend to wear out Humans, much like Cars. Ask Lord Opti how he knows.
Anyway - On that last point, a standard B234/5R with TD04 (all standard) in an Omega would make a modern 530D shit itself..... after a remap, and it's bye byye!