Based on my experience and the criteria you mention, you will find the Hundai Genesis meets your requirements in terms of engine performance, reliability, and comfort.
Acceleration is very good with the V-6, even better with the V-8, and highway gas mileage is pretty good for such a large car (I consistantly get better that the EPA rating on long trips). I own a V-8, and the easy access to power at any point on the rev range for merging and passing makes the car fun to drive.
Reliability is good, according to both consumer surveys and my personal experience.
Finally, the car is very comfortable on long trips, with a smooth and balanced ride and quiet cabin - I have had to make several 2,000 mile trips over the last year, and always arrive fresh and relaxed.
A bonus is the large amount of headroom, the reason I bought the car, and what I think is an attractive cabin with nice materials - YMMV, of course.
The only thing you may not find acceptable is the handling. The Genesis is not a sports sedan, but more of a GT car, tuned for comfort. The ride is not floaty and disconnected, but compared to the Maxima and 328 you have owned in the past, or the Mazda 6 you recently drove, you will find it less responsive when pushed hard. You will need to do a test drive to see for yourself if the handling balance is what you want in a car.