Repair or replace rusty rear sub frame?

[SilverTeg]

Hey guys, looking for a bit of advice. To my horror I discovered a large rust hole in the top of the rear sub frame ahead of the camber arm mount on drivers side on my 07 CL7. The passenger side and all the rest of the sub frame that I could inspect easily looks pretty solid, but given the extent of this rust patch I expect that if I drop the sub frame I'll find more. Have you guys tackled similar and have you repaired or replaced your sub frame with damage like this?

 

[JezUK]

Oh man, I hope I don't find something as bad as this.

I'm currently Lanoguarding my car and am at this very moment about to start doing the rear (I did the front a few weekends ago) - I've been told that the rear is generally worse than the front and I'm already calling my welder to come and do some plate work along the driver's floor panel..... Wish these great cars had better rust protection.