OK That makes is more clear.
The sub frame IS different in that besides the exhaust hangars, it also is significantly heavier (duty and physical weight); however they don't often fail, so you could stick with the ti part (although weight savings over the rear wheels is NOT an issue with the ti)...
As I recall, the M Trailing arms were also significantly heavier (duty/weight) than the ti ones as well. These only fail when they meet inanimate objects at elevated velocity.
I stand by my original suggestion: Use the M roadster or M coupe rear sub frame/diff/trailing arms/brake assembly complete when doing a swap.