It's not the test I would have run but it certainly beats not testing it at all. This is an encounter you might actually run into if the DMG advice is followed, so I admit it's a valid example to test your theory.
Maybe that's sufficient to consider the theory proved.
Actually, it was a group of foes (and the PCs were level 22, not 21). 3 lower level, 1 higher level, and the BBEG. The other foes died in rounds 6 through 11. Then, it was just a long grind on the Ghoul King.
http://www.enworld.org/forum/d-d-4th-edition-rules/252458-forked-thread-so-about-expertise.html
Btw, I never got to run the Kruthik encounter that I mentioned in that thread.
I will run an encounter with the Kruthik Hive Lord plus 1 Kruthik Adult and 1 Kruthik Young and 2 Kruthik Hatchling Minions (total XP 862 or about 3 levels higher as per the DMG guidelines, just like the other encounter although slightly weaker XP-wise) against these same PCs at first level. This encounter at first level seems more like a TPK than the one at level 22.
However, I did run the following just on Saturday against 6 2nd level PCs (and note, I do have some house rules that give PCs a few go to the well abilities, so our PCs are fractionally more versatile than normal ones):
The Kruthik Hive Lord plus 3 Kruthik Adults and 5 Kruthik Hatchling Minions (I used my tough minion rules where minions roll damage and last an average of 1.75 hits instead of 1, i.e. they have a 50% chance of dying on hit one, a 25% chance of dying on hit two, and a 25% chance of dying on hit three). 9 monsters vs. the 6 PCs. The XP was 1335 (since my tough minions last longer and average more damage, I double their XP), but even without doubling it, it was still an n+4 encounter. So, not exactly an apples to apples comparison.
It was a tough fight and the Rogue fell multiple times and I think the players thought it was going to be a TPK at times, but it turned out ok. It was a lot more serious of a challenge and scary to the players than the 22nd level fight (which was just grindy because the PCs have so many options at that level to handle bad situations), but the level 22 fight was only n+3+ (course, the player playing them was not familiar with them and the sample PCs were not optimized either, so it was probably tougher than it could have been).
