I believe Jim Beam Rye and Old Overholt are about as similar to one another as Blanton's and Eagle Rare (also the same mashbill). Or Ancient Age 10-Star for that matter. What comes out of the still and goes into the barrel is only one factor (well, okay, one set of factors) in what the final product will be like. Which of all the differently-matured barrels are selected, and their proportions in the final mix, is what provides most of the variety the distiller (or bottler) can offer.
Beam inherited a particular flavor profile for Old Overholt when they bought the National Distillers brands and they've done a pretty decent job of maintaining that profile separately from their already-existing self-named rye whiskey style. The version of Overholt that ND was bottling in the late '80s when they sold it was quite different from the one that made the brand so famous both before and after Prohibition, and that older version was the only 100-proof (BIB) Old Overholt marketed, AFAIK. I DO believe Knob Creek Rye more closely resembles a 100-proof version of the current 80-proof Overholt than it does the current 80-proof Jim Beam Rye. Best way to tell is to have some of all three and reduce the KC's proof down to 80. Better yet, get someone else to do it for you so you can blind test them. That's what a good bourbon bar is for.
Also, FWIW, I think Knob Creek Rye is one of the best I've tasted. And for the same price it beats the pants off of Sazerac 6-year-old 90-proof.