I have stock Pzero Neros on my car and roll empty trunk all winter. Just don't drive like a moron and you'll be fine.
More weight back there can also be a bad thing. You want more steering and brakes, not acceleration.
Same here. And I have a 45 mile each way commute every day. Never have a problem.
As far as cinder blocks and the like in the trunk.....I hope you also wear a helmet and flak vest for protection. What do you think happens to all that stuff in the trunk if you do skid and come to a sudden stop? The seat backs aren't going to do much to protect you when you are at 0 mph and the weights are still moving at 70 mph.