The maximum size allowed by NASCAR is the 830 CFM Holley. The actual carb used by the top NASCAR teams may look like a 830, may have the correct dimensions for a 830 Holley-----but the are not a 830 Holley.

Yes, a P7 can use more than an 830, but how much more depends on RPM the engine is running. When we used our 358" P7 in an 2300 lb. oval track car, we used a highly modified 800 Holley---and it was good to about 9000. The cylinder heads are highly efficient, so it doesn't require as much carb as less efficient heads do.