I just bought a Demon 750 to put on my SBC 400, and it come with 4 corner idle screws vs the 2 on my 3310. What can I expect from the demon (Performance, Idle, Response) compare to the Holley Carb.

Just a bit more tunability.

Unless you have a lot of cam the 4sided idle circuit really isn't needed.

The 1 and only 750 Speed Demon I tried with a cam that size was very Lean to the point of shutting off at cruise on the transition circut.

Set the idle screws out 3/4 from seated and fire up the motor. Let it get to operating temp and then turn the idle mixture screw in or out 1/8 turn. Which ever one makes it run better. Then go and turn the remaining screws the same amount.

Normally with 4 corner idle circuits vs. 2 corner idle circuits you can fine tune the carburetor better to the engines requirements. This should give you a smoother, & cleaner idle, as well as better throttle response.