Well, I'm running 235/60R15's on my Malibu and I can say that they make a huge difference in how the car handles and grips the road.
Where are you looking at the disc conversion from? I've seen a bunch but I wasn't ever looking at going larger than 16" rims when I was considering it.
Oh I still have the Hotchkis rear coils for it if your interested. The 4dr was just too much for them, but I bet they'd work much better on the elky as long as your not using it to haul stuff. /forums/images/graemlins/thumb.gif