My friend got a set of IS300 rims and it fits fine except.....
The middle of the hub on the IS300 rims is smaller then the P5 so the rim can't slide all the way on. You will have to modify the center section of the IS300 rim (any machine shop can do it for cheap) and i will fit perfectly. As far as the offset, I can't answer that but I can say that it looked good too me!
Dave