The notch is there for clearance for the three bbl carbs; Holley and the ultra rare Carter AFB.
You can increase the total cfm's by 60 with the removal of the choke plate. This can easily give you another 20-25 hp on the 850 cfm and 15-20 hp on an 800 cfm. If you do this, you will have to increase the jet size on the primary side, David.