Firstly if you are going to pull out specific monsters as examples I have every right to pull out examples of PCs who make them not a problem. That is part of the point - DM discretion is important, and chosing the right mosnters for the party is important, even if that means changing a pre-written module (which seem to have "pick a monster" approach all to often so its not like it matters).
(Btw the War Chanter Bard PP can easily generate +9 attack bonus for every ally for a turn at 20th level... talk about situational changes to the balance of the game.)
Bottom line: If you want to maintain the scaling model and encounter design system the correction of 1 per tier is indeed necessary. PC's are going to struggle massively against L+6 monsters in epic. L+6 monsters in heroic are challenging but not overwhelming, something changed between tiers since facing L+6 monsters is part of the game. Anecdotal scenarios of how some pc's can still compete with L+3 monsters isn't a valid argument to the contrary. L+3 is not L+7 and for every situation where a pc is strong against a monster there's a monster that is too powerful for them to deal with. The sorrowsworn level 28 leader can conceivably be in a battle with level 21 or 22 pc's.
See you are assuming that the Encounter Design guidelines in the DMG are without flaws. I would suggest that the first DMG has many flaws in it wrt mechanical matters. I would go so far as to suggest that problems are seen in high paragon and epic when following the guidelines presented in the DMG because the guidelines that went to print were simply not tested rigourously enough to detect the flaws; because good DMs were making good encounters for their PCs - the bit that isn't covered by the crunchy stuff.
Just think of the problems with Skill Challenges, and still existing problems with Skill Check DCs in 4E created by the gap between Trained and Untrained being potentially huge (+10 or more in some situations). I have a character whose response to skill challenges is to largely do nothing for example (though that's more an issue with LFR skill challenges and DMs).
In essence the Encounter Design rules work well in Heroic Tier and early Paragon, after that they start running into problems.
Further to this you keep holding up the math of monster design as if it is ideal as well, yet few of the monsters in MM1 conform to the math we are given in the DMG - most having lower defences than the DMG predicts in at least one area, and the DMG is terrible at predicting AC, which is usually higher in the MM.
So what you have is an argument about the math of the game using systems (that is the ones in the DMG not the ones WotC actually used) that are flawed as its foundation. Not a good place to start tbh... much better off looking at game play and seeing how things really work.
In encounter design you should be taking your XP budget and looking at monsters that have defences around +10 on the typical attack of the party. Selecting monsters that the party needs to roll 7-8 to hit without effects will lead to an easy fight. Selecting 9-11 will lead to a moderate challenge, and selecting 12-14 will lead to a tough fight. (Those are "most of the party" ranges, individual PCs may need higher or lower values.). Grindspace, as its called, is something to be avoided, and following the DMG guidelines on encounter allocation of monsters is a good way to create it. As a rule you should be using monsters that are +/- up to 2 on PC level. Elites should be typically PC level, maybe up to Level +1, and Solos should be from Level-2 with buddies up to level +2 at most and alone depending on their defences. Similar evaluations on the to-hit chance of the monsters can be done as well.
None of this is well discussed in the DMG it just makes wild statements with little to no guidance on the application - reinforcing my belief that it was heavily tested for Heroic tier, but basically not tested for Paragon or Epic.
It will be interesting to see what DMG2 has to say about encounter design if anything.