Overall, I like the car, and some of the comments you bring up are totally valid - you are not the first.
If I recall your photo in the Pictures thread or somewhere, you are not a small guy. Therefore, the side bolsters and low seating position do not work well for you and they don't work well for me, either.
It's too bad, because in restyling this car, they could have done a great job without swinging the pendulum too far over from the 1997-2003 model run. Here's my list.
Seats:
They should have kept the seating profile (flatter) and the softer touch materials of the previous model run. They were comfortable enough and there was nothing wrong with them.
Backlite and roofline:
Again, they could have hit a "middle ground " in the roofline and backlite department between the last model run and the current one. Sometimes, and extra inch of height and a "not as flat" backlite could have done wonders for the feel of the interior, particularly the back seat.
Dashboard:
The materials at the sides of the console and underneath the dash are indeed rickety. These could have been nicer, as they are even in the less expensive Malibu or an Impala.
Front end:
Again, they went overboard with the "cat eyes" in the front. They should have been more G6.
Grand Prix's strong points are:
the powertrain, the overall rakishness of the styling, the handling compared to other W-bodies, and the overall shape and cockpit feel of the dash, and the longevity of the car.
My only gripes are above where they got too far away from the good points and 1997-2003 run and made the car less user friendly to a load of 3 + people. It's these gripes that have made me pause about buying one ....while I paused at the 1997-2003 because of those stupid effed-up "pumpkin" lights in the rear...good riddance to that stupid styling feature.