Darren: Your questions would take an entire book to give you a straight answer, but the bottom line is ride quality is directly proportional to wheel deflection and travel. And contrary to many misconceptions, a soft suspension doesn't mean a bad handling car in most cases. If a suspension is stiffened, it's done beacause a compromise had to be made, i.e., solid rear axle in a Cobra. Why? Because a lot of wheel travel would mean a lot of u-joint angle with that short driveshaft, which would result in frequent u-joint failure.
The IRS setup in the rear improves everything in the suspension department: ride, handling, trunk space, originality . . . providing it's done correctly. Wheel travel can be increased, suspension can be softened (give a lower ride frequency), unsprung weight is reduced, camber gain keeps tire patch flat on ground, sway bars provide roll control, and you can redo your trunk for more space like was originally done.
To get a real lesson in high unsprung-to-sprung weight performance, drive an empty pickup truck--not too fast--around a curve on a dirt road with a washboard surface and notice how the rear end wants to pass the front end.
Get Fred Puhn's book
How to Make Your Car Handle to learn about this stuff.
Simple
