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.
The sub frame is a mass produced cost conscious piece designed for a road car on 2004 tires - the force produced by more modern tires and improved suspension dramatically increases stress on the chassis.
This example by opera performance for an s2000 gives a good idea of a modified sub frame - however if you notice the large dimpled holes everywhere - this is an old trick to reducing weight and increasing stiffness as it's very hard to deform a radius-ed circle
JUN Auto's version has alot more holes and showcases the seam welding most of these options have too
However there is a level beyond this and it used to be alot more popular, full replacement - especially if you have lightened the engine, or running a 3 mount setup
This version from Beatrush for the WRX is a good example and probably shaves quite a bit off while still performing well.
This is a full road race and drag race piece for the GTR and it's 20 pounds lighter and offers superior geometry for lowered cars and steering arm angles, it also vastly outperforms the factory piece.