LA 318 advice

No it won't. As long as he keeps duration down, it will not. If he uses something like 216 @ .050 on a 108, he'll have a barely noticeable idle in a 318. You telling him that is gonna scare him away from what he needs. All slant 6 cams are ground on 108 and 109 LSAs. Do they idle rough stock? No.
Never said it couldn't have a mild idle on a 108, I said overlap is responsible for rough idle and the two main factors are more duration and tighter lsa, so if you go with a tighter lsa with a smaller duration you get less overlap smoother idle, it depends on the overall overlap not how you accomplish that overlap was my point.