Thanks for your comments everyone. I was just amazed at how much of a difference it made so it definitely eliminated some flex somewhere. I'm very happy with the result but was worried about what might happen when I take the car on some rougher roads. It's good to know that nobody seems to have had any problems.
Of all of the things that I've done so far in the car's mid-life suspension refresh: new KYB struts on all corners, new sway bar end-links on all corners, new sway bar bushings on all corners (new oem rubber in the front, polyurethane in the back), new aluminum front control arms with the solid mevotech rear bushing, installation of a strut tower brace; the AVO braces probably had the most noticeable effect on the precision of how the car handles through curves (on Highway 99, Vancouver to Squamish and back). Before that it was hard to "feel" the curves and the rear would seem to wander. The tail still feels like it sticks out a bit in curves at higher speeds but it's much more predictable and consistent. (Would a thicker rear sway bar help with that?) I suppose it's possible that the AVO braces were the last piece of the puzzle needed to get the car's suspension back to a respectable state and that's why it felt so dramatically better.
BTW, the rubber subframe bushings on my 2008 2.5i are actually similar to to the OE rear bushing on our front control arms in that they have voids in them. They looked OK but I do wonder how much they have softened up with age. I purchased the car used (with about 60,000 miles on her; now she has about 130,000 miles) so I never experienced how the car handled when new.