At Idle.

So, does a performance engine at idle require a richer mixture?

There isn't an answer that you can apply to every engine, and the term 'performance engine' isn't really that descriptive. In general the idel behaviour will be affected by the cam profile used, and as the cam profile becomes more aggressive (more lift, duration and overlap), you will need to raise the idle speed and you may also find that the engine will not idle happily at a stoic AFR. I'd simply start my tunign around stoic and if the engine doesn't run well then I'd try a little richer. Generally a target of 0.90-0.95 lambda should be enough as if you tune too rich you'll risk fouling the spark plugs.

Thanks Andre!.