For what they are, they handle very well. I can speak of high-torque FWD cars because I've had decent wheel time with three of them (Aurora, Regal GS, SSEi). Even with 280lb-ft of torque right to the front wheels, torque steer is not a factor. You notice it under heavy acceleration, sure, but its never unmanagable. I can recall one or two instances when it got a bit out of hand, and that was more a combination of my stupidity, damp roads, and either a railroad grating or those slick road marking stripes.
The worst handler of the three would be my Olds because its so g@($@$n heavy. Both the Bonnie and Regal are lighter by a few hundred lbs, but not too light. Also, I would argue the G-body is the best fullsize platform currently in existance in terms of rigidity, strength, and overall strength. Ride quality naturally depends on the car, but even a car as old and potentially 'loose' as my Aurora (frameless windows) doesn't shudder as bad on rough roads as newer cars.
And though I haven't significantly driven a LaCrosse yet, if its even at least as quiet as the Regal was. Wow. Because that was a pretty quiet car.
The '00+ Impala is on an updated W-body with a 110.5" wheelbase (1.5" stretch from the '97 Ws) and the '04 GP and '05 LaCrosse share that. I would explain the smaller rear seat room with the faster rooflines of the Pontiac and Buick. More sleek = less headroom. This also accounts for the moonroofs in these cars retracting up and out instead of into the roof.
Oh, and thanks. I my Aurora.