Right, but if you installed longer plugs regardless of how small of a gain you would see your still filling the combustion space with more material so you would lower your CC's regardless of how small.
Things like this come up from time to time on Forums. Discussions and deciding between what "could" be done Vs what "should" be done.
Technically, you're correct. Even the tiniest amount of spark plug protrusion into the combustion chamber will effectively reduce the size of the chamber and result in a tiny increase in C.R.
Assuming we had the ability to accurately measure that small difference, and all things being equal, there would be a tiny increase in C.R. However, the downside far out-weighs ANY benefit.
As already mentioned by MIZ28, if any thread is exposed to the combustion process, it'll/they'll quickly begin to glow red hot and cause detonation. It's one of those MANY things that the best engine builders check for before the heads are bolted on.
So, even though you are technically correct, it isn't something you'll find as being recommended by anyone. In fact, it's something I've seen in the manuals as something NOT to do. So it's not recommended unless the guy making the recommendation has a grudge against you, LOL
Would C.R. increase? Technically/mathematically Yes.
Provability is dependent on our ability to detect it using our equipment. I'm betting NASA could though.
Would any power improvement result? No, not within our ability to measure such tiny changes. Even then back-to-back dyno runs don't show the exact same results every tme.
Would doing this be of ANY benefit? No, just the opposite in fact.
Is this something that is recommended to be done or at least done by some guys? No and No (unless done unknowingly)
Are there any more questions I should have asked? No, you asked too many already.