Hey Mikey, I think I can address all your questions as I've tried many iterations of suspension and alignment on my car and tested them in every condition imaginable...street, autocross, and track. Sorry if this response gets long.
Earlier this year I installed Ground Control coilovers (my third major suspension setup since owning the car). I had the kit custom-specified with different sleeves, spring free lengths, and rates from the "off-the-shelf" kit for the Contour to suit my competition usage and adjustability objectives for the car. Ultimately my point is that my perspective is entirely from a performance standpoint, and as such my car is only moderately lowered and running very stiff springs front and rear.
I too had a 21mm rear sway bar prior to coilover installation, and it was a very good improvement to the nimbleness of the rear during transitional handling, especially with some toe adjustment to dial in exactly how easily I'd want the rear to break away. The benefit for street driving is that the bigger bar doesn't affect ride quality all that much.
When I went to coilovers with high rear spring rates, the bigger bar was too much. So much roll stiffness comes from the spring (as it should with strictly performance in mind) that the big bar wasn't helping with body roll and handling balance so much as it was transmitting upsetting road inputs from one side to the other and creating a choppy, unpredictable feeling in the rear. Adjusting the rebound damping couldn't fully cure it. I decided to remove it (well, disconnect it at first). The rear immediately settled but the car then just pushed like no other with only the front bar. So I disconnected the front bar. Balance went back to neutral with some toe compensation and actually felt pretty damn good. So much so that I actually drove a few autocrosses with both bars disconnected. However, the steering turn-in precision was all gone. The front would, for lack of a better way to describe it, squirm for part of a second before taking a set, and it was enough to be very disconcerting when driving at or near the limit. This may be what the previous poster was describing as the "loose" feeling. Anyway, I also wouldn't recommend going with no front sway bar, especially with "streetable" spring rates. I wish there were a smaller bar option for us though.
To wrap up this saga, I ended up reconnecting my stock SVT front sway bar and removing the 21mm Roush bar I had to replace it with the stock SVT rear bar. I'm fairly happy with the handling now, although the front is still a little too firm in roll for the open diff. So now to get a Torsen in the off-season so I can actually pull through corners...
