Poor idle and off-idle performance after distributor swap

So if I read this correctly, you are saying the old system had the same problem. Was there any part of the Old system, reused in the new system?
Have you tried putting your timing clamp on #6 plug wire, to see if it is doing the same thing.
And I suppose you have tried turning the clamp both ways?
I assume you have checked the timing-chain slack?

I think that the slot is not the problem. While any variance of distributor degrees translates to double that on the balancer, that is barely perceptible to you in the cockpit. And I challenge you to feel a power difference below 60 mph.
But if all you have is a 5* variance, with no wild strobes, at any particular rpm, then that rules out a polarity issue. And that points to just a mechanical issue.
Starting with the timing chain, the cam end-play, the intermediate shaft, the oil-pump drive hex, and ending at the drive-slot. Of these the chain-slack is the most likely to cause an issue.

Is there the same 5* variance at 3500rpm?
this would rule out the advance springs and the V-can. And should rule out the chain and cam. That kindof leaves just the I-shaft. If it jumps up and down, it will change your timing, by virtue of the angle drive. The oil-pump load should keep it down. But if the pump load varies, and the endplay to the D is excessive, I imagine it could move up and down.
But it is hard for me to imagine the oil-pump load varying with any degree other than constantly increasing with rpm. I suppose if the hex was worn excessively, the I-shaft could be slipping in the pump.
One thing I would check is the wear-pattern on drive-tang,of the old D. This will tell you how deep your D is entering the drive-slot, and that could help to understand if the I-shaft is migrating up and down.

Honestly, I got nothing for you, but just trying to help.