Nice to find an older post where the problem was solved and explained thoroughly.
First off.....I don't think a rough idle is "normal" on any stock Focus. Of course when you modify the engine in any way, all bets are off.
Finding most vacuum leaks is fairly easy. If you suspect one (rough idle is one symptom), with the engine idling, lightly spray around the intake manifold and all the vacuum hoses.....with carb cleaner. If there's a leak anywhere, the engine will speed up. Avoid spraying the spark plugs and wires, as well as the exhaust manifold area.
You can pinpoint the exact leak with this method.....carb cleaner is flammable, so be careful.
I'll keep my guns, my freedom, and my money.....you can keep the "change".