To be honest, those seem to be pretty big valves to try to squeeze inside a .030" over 390 bore. And even if, they somehow do not contact the cylinder wall, I have to wonder if the shrouding of the larger valves will actually be of any real benefit. By comparison, the valve sizes on my TFS heads are 2.19" and 1.625", which is actually smaller than stock CJ exhaust valves, but with larger intakes. I have to believe that TFS thought a better exhaust port, with a smaller valve, was a better choice for smaller bore FEs. Since the Pro Maxx are basically an off shore bootleg copy of the older Edelbrocks, I have to wonder if they even tried and compared the larger valves to see if there was any real advantage, in a smaller bore, or just did it to say they have bigger valves, even if they are of no actual benefit.
So .005 radially isn't much different from a TFS, but as said before, depends on valve spacing with the Pro Maxx heads. Shrouding is interesting, to be honest, the only time I have seen shrouding as a
primary issue, meaning more important than port and valve job, is in the chamber with the small chamber 352 heads. With that setup, Tommy T gained quite a bit of flow by opening the chamber. Now, not saying cyl wall shrouding isn't a thing, of course it is, but the ports on some heads are so bad that port performance likely makes more of a difference at normal valve lifts.
I do agree though, at some point there are diminishing returns. Assuming these are the heads he is planning to use, I'd bolt a head on and measure clearance to the bore, on each valve, at max valve lift. If they don't hit, run them, a 330+ CFM head will still do well through most of the valve travel. Although again, no idea on that Promaxx, but on those iron, the exhaust will walk toward the wall until it hits, so it may not be as bad at lower lifts
Last comment, I don't think TFS went small because the exhaust was good. I have ZERO proof, but I think they wanted an awesome intake port, and then ran out of room. The TFS is one of the best out there, but it is pretty lopsided on the exhaust port
I would however pour the Pro Maxx intake port, if it's very big, I would pick an intake lobe and lobe timing to pull harder on a port if street/strip, especially with a 445