"Why wasn´t this tried if there were no such obstacles, Jank?"
Simple. The USAAF was wedded to the .50. The P-47 did just fine with 8 x .50's in ground attack as most targets succeptible to 20mm damage were also succeptible to .50 API damage.
Later in the war when the P-47 assumed primarily a ground attack / close air support role in the ETO, existing Thunderbolts were pressed into action. Why go through refits? The Germans at this point in the war were playing defense. The Allied advantage came from sheer numbers of aircraft. There was no reason to rethink a problem that didn't exist to begin with. We're talking here about making the P-47, which was already highly effective, even more effective and not about correcting a noted deficiency on the part of the P-47.
The fact that it was not done is not evidence either that it could not have been done or that it was not feasible due to recoil.
"I showed that the recoil of this layout exceeded typical ww2 Fighter A/C values by much."
So what. I'm sure that in 1940, had you been in the USAAC, you would have told Alexander Kartvelli that his idea for an 11,500lb behemoth, armed with 8 x .50 cal machine guns "exceeded typical ww2 Fighter A/C values by much" and that therefore, it could not possibly be a success and was a waste of time and money.
Well looky here (the prototype ended up weighing even more than his 1940 proposal for the XP-47 design):
Republic XP-47B Thunderbolt
The XP-47B prototype (40-3051) flew for the first time on May 6, 1941, piloted by Lowry L. Brabham. This was only eight months after the order had been placed. The XP-47B was the largest single-engine fighter built up to that time. At a loaded weight of 12,086 pounds, the XP-47B dwarfed all previous fighters, being almost twice as heavy as most of its contemporaries. On the first flight, the pilot was forced to make an unplanned emergency landing because of a leakage of exhaust fumes into the cockpit. Its eighteen-cylinder XR-2800-21 radial engine offered 1960 hp at 25,800 feet, and gave it a maximum speed of 412 mph, 12 mph faster than Kartveli had projected. An altitude of 15,000 feet could be attained in five minutes. Empty and normal gross weights were 9189 pounds and 12,086 pounds respectively.
As to the Xp-72, you said,
"This plane also did not reached combat status, the proposed armement layout was not accepted by official standarts (no firing in flight tests were acomplished), so it totally remains speculation and not fact."
Well, you are right, of course, the plane was cancelled after having been ordered to be produced with 4 x 37mm guns. The fact that the 4 x 37mm layout was ordered into production by the USAAF does, however, constitute an "official acceptance" of that layout. It is also speculative to say that no flight tests were performed with that layout. I would just say that it would seem to be rather odd that in 1944, given what the USAAF knew about fighter design and performance that it wouldn't have determined that this layout would have worked. Of course, we'll never know and I'm not going to argue the point further because it is rather speculative. I wonder if Tony has any thoughts on the XP-72.
I will, however, refocus you on the point raised above in my post:
"This discussion has broken down into a complex, myriad of equational monstrosities and as such, the forest is being lost through the trees. The fact remains that the Hawker Hurricane handled four six 20mm's just fine. No evidence has been produced that the ensuing recoil of that set up caused stability problems.
Ergo, a 6 x 20mm set up in the Thunderbolt would not present any such concerns either.
The production run of 4 x 20mm armed Hurricanes was large. Any recoil associated problems would be peppered throughout the RAF's records and the numerous pilot accounts that have been memorialized to writing. Can you produce a single one?"
You point out that the Hurricane was primarily used in the air to ground role with the 4 x 20mm configuration and that this is evidence that it didn't work in the air to air realm. I would point out that the P-47, later in the war also assumed primarily an air to ground roll. Are we to conclude that this was because it was found that a compliment of 8 x .50's produced too much recoil to be effective in the air to air realm? Of course not.
You know very well why the Hurricane was relegated primarily to air to ground operations and it had nothing to do with the unacceptability of the 4 x 20mm layout. In war you attempt to direct resources efficiently. If there are planes with better air to air performance (Spitfire), you put them there in that role. The Hurricane was obsolete as an air to air resource whether with 8 x .303's or 4 x 20mm's.