Sorry Isaac, you must have driven a duff 3.0 
There is a very noticable difference between the 2.5 and the 3.0. Oh and the 3.0 is just as smooth. 
My first omega was a 2.5 97R. It was pulling to the left (but that’s another story), and during my discussions with the technicians at Vx they suggested that I drive another car to see if it behaves the same – and off I went with their top techie in the Service Manager’s own 3.0 Elite – which had around 25,000 miles on the clock iirc. We went up and down the M1 from Brent Cross to J2, but I was not able to convince them that cars should go straight ahead. This was back in 98.
Later in 2001, when I was looking at replacing the 97R car, I test drove a 3.0L X-Reg Elite (early facelift), silver with black leather (my dream combination), with only 1,000 miles (!). It was a beautiful car, but the asking price was £24,000 which I thought was too high. I ended up buying a 2001Y CDX 2.6 with 6,400 miles for much much less.
On both occasions, the 3.0L cars did NOT feel substantially more powerful (though they were somewhat more torquey), but the 2.5 and 2.6 seem much smoother when accelerating steadily. This is my experience, anyway…