Whats involved replacing IRS with a solid diff...

[Michelson4301]

The tyres lean in on Holden IRS when you gun it or load the thing up, causing more negative camber and less tyre contacting the ground

 

[VTSSDUDE]

I would say camber kit and see how you go. From what I heard you are better off using third party kits then Holden kit itself.
I also had a mate in a VT Series II SS, had his back tyres wearing out. He had the camber kit put in, fixed it. He didn't go Holden either.