I've definitely thought about getting a spare subframe and finding ways to make it lighter and possibly add stiffer bushings. I'm not quite sold those rigid collars do anything but may try them anyway so i know for myself.
Some thoughts on sub-frame since I have researched a bit about TSX chassis, comparing it to more recent cars like TLX, Accords and BMW to see where it can be strengthened.
For sub-frame, I actually would rather add weight to strengthen it then taking out weight. The weight there is good mass - since it sits low, helps noise/vibration and improves handling. Besides, the factory design looks lean already. Not saying it is impossible but for sure not simple.
I am not a structural engineer so take the following with a big grain of salt. From the various crash photos I studied, the front and rear beams of the boxed sub-frame are quite strong. The other two beams obviously can not be be because they need to bend to absorb energy. This might be an area for improvement.
Another weak spot I feel is the two frontal attachment points of the lower control arms. Here two brackets are welded to the main assembly. This is one of a kind design and later models all have the attachments built into the beams. I want to try the Alutec 2-pt bar that ties the two points, but ground clearance concern prevents me from doing so.