I have a hard time understanding how .5 additional compression would make any discernible difference in performance or ignition timing. I believe that you know what you you’re talking about I’m just having trouble understanding it.
Because I gave a really short answer, trying to answer your question without distracting with all of the engineering reasons behind the statement.
The original compression ratio head is very forgiving when it comes to spark advance. Your spark lever can provide up to 40 degrees of change and most people can find the sweet spot somewhere less than that. Around 30 is usually pretty good, but 40 will not hurt too much of anything unless you are working really hard climbing a hill, especially at low RPM.
The Ford Instruction Book advises on how to adjust the spark lever. Most drivers just "pull it all the way down and leave it there". That's not what the Instruction Book says to do, but that's what's done. And the engine will mostly survive.
When you go up to a 5.5 CR head, you actually get a nice boost in power. It meets your needs. The most power on this head is obtained closer to 26 degrees of total advance at higher RPM (like 2,000 -- less at lower RPM). 30 degrees is too much and 40 is WAY too much.
The 5.5 head is less forgiving of excess advance than the stock head. Part of the reason for this is that it is a flathead design. Later head designs (like Chrysler Hemi or other "valve in head" designs) can handle a bit more advance.
So if a novice driver runs the 5.5 head "with the lever all the way down and I never touch it" they are likely to get some serious knocking and pound out their bearings under heavy load, especially at lower RPM.
Now when you go up to the 6.0 head you get even more of the above. Peak power at high RPM is obtained closer to 22 to 24 degrees, and this head will quickly start knocking under heavy load with too much advance. Climbing a hill it is common to have to fall back to ~16 degrees. If you don't, it WILL knock. You will mess up your rods and mains. 40 degrees will actually give you less power than at 22 to 24 at 2,000 RPM. "Too much of a good thing."
The added power from the added 0.5 compression ratio is small. IMHO the downside risk of having it knock if you don't manage the spark advance properly is not worth the little extra power you get.
Therefore I advise most people to get the 5.5 head, where you get the biggest bang for the buck, compared to a stock head.
And even then, I tell them that you MUST not advance the spark lever all the way down if you time the engine to exactly TDC like Henry's procedure has you do. If you want to intentionally set the initial timing some degrees after TDC, then you can do that and go a bit further with the spark lever. It's the total advance that matters. All of my numbers above are total advance (initial plus spark lever advance). Most guys experiment with the spark lever to find the sweet spot for their engine and the initial timing that they have.
One of the reasons the Model A has survived for as long as it has is that Henry's original design was very forgiving. That head might not have produced as much power as we want to see today, but the engine could survive misuse and abuse with it.
Many of us today cannot hear the engine if it knocks, due to age and due to other road, wind, and engine noises. We can be running down the road knocking like crazy and not hear it, or not recognize it, so not adjust the spark lever up. My guidance it trying to protect the drivers from themselves a bit. Nothing is perfect. If you don't want to worry about it, stick to the original CR head. It is much more forgiving.
