ISL Exhaust Manifold Leak - How far can I drive it?

I had a exhaust leak on one of the manifolds on my Cat engine. I could not see where it was leaking, but I could hear like a tapping sound. I still drove it for at least 1000 miles. Crawled under the engine with a flashlight and I finally saw where it was blowing black smoke. I took it to Freightliner Las vegas and had it repaired. I notice I have lots more power and better fuel mileage.