Here is what I read, about Edelbrock Performer intakes;
The castings may, or may not take a centering correction, depending on how far off the runners are.
When they cast the cores, typical shifting issues don't occur between the paired runners, but rather, between pairs on the entire bank.
So, when you look at the sides, measuring them, on core shifted intakes, they may look something like;
[][]_____[][] on one side and
[][]______[][] on the other.
We're not talking about a huge difference. It only takes 3/16" to throw an intake off that needs re-centered.
The reason is because of the thin, divider wall between runner pairings. The walls on the outsides can be corrected easier than the divider, because you can't move more than 1/8" from the inner walls, to help it even up.
Another issue you may see with core shift is an entire bank not aligning with the bolt holes drilled through the intake. They set the intakes up on a jig and CNC drill them for bolt pattern and alignment dowel holes (if yours has the holes).
Most new intakes that I've seen have them. The old ones didn't, because it was assumed that you would center and check the intake. Manufacturers these days are under the assumption that casting quality control is higher. On some cases, this is true, with core shift on newer engine blocks and cylinder heads. Even the walls of the sand castings are smoother, but you never know unless you measure the areas of concern on any part, new or old, for yourself.
What happens when either the bolt pattern up top, or the index peg holes are drilled too far forward or backward is that you get alignment issues on either one or both entire cylinder banks. If the ports on one side have shifted forward or rearward, this may not show up on a measurement check between sides, but you should still measure for paired runners spreading from the other pair on that bank.
Along with the rocker shaft issue that moparlover pointed out, pushrod length, deck height and cylinder head decking on combustion and intake sides all have some play in it.
I would assume that the engine runs rough, considering that one side is inherently making more power than another.
One other issue to look at, as you've suspected, would be an intake gasket leak on one bank. If the intake wasn't milled properly, one of the heads varies in it's mating surface, compared to the other (milling done on combustion or intake plane side) or if the cylinder head casting on the mating surface of one head is simply not sealing, you could have a vacuum leak.
My guess is that you have a combination of issues on one bank, that a good composite gasket set and centering job can take care of, without grinding.
To center the intake, pull it, clean everything, lay the gaskets on the heads and make sure that they center on the runners. They usually do.
Flip the intake over and lay the gaskets on the intake runners. put the bolts in, backwards (easier, as it is upsideown) to show you where the gaskets will want to sit on the intake, when it's bolted together.
This should show you core shift. You can see interference from the gasket to intake runners on one bank if you have pairing shift. If not, you are ok with spacing, per side, but the intake still may have core shift and that will show with the next step.
After you've determined that each side is correct to itself, mark the centers of the thin divider walls of each runner pairing, do the same on the heads, so that you can see both sets of witness marks when the intake is on. Plop the intake on, with the gaskets in place on the heads and no sealant.
Line one side up and look at the other. If it is off, the distance that it is out is exactly half of the distance you move it, in that direction. If it's further than 1/16" you may consider removing material from the left side on one bank and right side on the other bank, in whichever direction you need it to move.
To be sure about how much material you should take off (only on the side of the runners that needs to move), make a witness mark on the gasket. If you have composite gaskets, be sure to index them on the head with the plastic index inserts that come with the gasket, then mark the gasket.
Make a second mark on the gaskets after the intake is corrected/ centered on both heads. Take the gaskets off with the intake again, flip them over and align the 2nd/ corrected/ centered marks. You should see how much the gaskets interfere. Corrections should be made to the gasket, where it overlaps the opening and to the intake runner on the other side that shows.
If your mechanic has one, a fog machine can show intake vacuum leaks if you have any. Sucks the fog into the leak. Starter fluid sprayed at the intake plenum mating surface on each side can also detect leaks when running. A cheap way to make a temp fog machine is to use a wide mouth drink bottle and put some dry ice and water in it so that you get a good amount of fog coming from it, then hold the opening near the intake, or cut a hole in the lid for a hose and set the hose end near the intake. The pressure from the c02 coming off of the dry ice will feed the fog out of the hose.
