Are wheels backward/forward compatible?

[lsxjunkie]

Anyone know what the offsets are for the GT 20s and/or Performance Pack 19s? I want to know if they'll fit on an S197 or conversely, if the wheels off a Boss 302 will fit nicely on an S550.

 

[SID297]

I've seen guys swapping them, but not much of a mention on how well they actually fit. I've also seen some shops using off the shelf bigs and littles with no issue.