First of all, the combination isn't that far off.

Performer RPM - great choice

Holley 850 Vacuum Secondary - Plenty of carb but not too much for a 440. If it's like almost every other 850Vac Sec carb I have read about, it needs tuning. Badly.

Comp XE275HL - Not a bad camshaft. I'd leave it in. Do take the advice to degree it in advanced. It was ground to be advanced four degrees if installed dot-to-dot and I could see going six. The advice to degree it in is right on.

2" headers - More suited for a stroker. I'd think they could be reduced in diameter to get more torque below the torque peak.

Distributor advance curve - speed it up if you're running a stock distributor. And limit the mechanical advance so you can run more initial. There is a lot of apparent power in the distributor, just waiting to be let out.

440s with open chamber heads generally do best at 38 degrees total advance.

Pistons 0.050 or so in the hole - this is typical for a higher performance 440 piston. Only the SixPack L2355F piston would be higher. I agree with JRR.....Don't cut the block 0.050. If you must cut something, cut the heads. There is no advantage to putting pistons at zero deck with an open chamber head like the 452s, or 346s, or 213s, or 906s etc. A stock 440 needs 106cc above the piston to get to 9.5:1. Your heads are probably around 92cc. There's 12 cc in the gap between the piston and top of the deck. If the engine builder bought a Fel-Pro kit with a composition gasket, that gasket can add as much as 12cc. So the engine would be somewhere around 8.7:1 right now. If it has a steel shim in it it's probably around 9.1:1.
I calculate you'll need to lose 0.042" from somewhere if running the thick gasket and if running the thin gasket more like 0.020".

I'd suggest putting it together taking care of all the details and trying it with the current headers. If it still needs more poop then maybe swap for smaller header size.

There is a lot of torque hiding in tuning the cam, carb and ignition.

R.