I know this thread is a couple months old but I thought that I would add some thoughts.
The conclusion you came to is generally correct for this scenario. Leaving your system @ 4 cores will give you optimal performance for the vast majority of programs/apps/games you use.
That being said, there are some benefits to disabling cores to achieve a higher overclock, but it is dependent on the software/game being used. Some (even recent) games were not coded with multi-core utilization in mind and are limited to using say 1 or 2 cores total of the 4 you have. Oblivion, for example, came out about 5 years ago and will only use at most 2 cores, but is a *very* processor intensive game, especially with all the additions and mods that have enriched it over the years. For a game like this, you *will* see a more dramatic improvement in performance by improving the clock speed on the processor, but the game will just ignore any cores past the 2 it's using. I personally have an AMD Phenom 2 B55 (quad core, unlocked 555BE) and use a 3.6GHZ 4-core profile for pretty much everything I do, but I set up an additional profile in my mobo's bios (it has 3 presets) to only have 2 cores active if I choose and clock them @ 4ghz. This does give Oblivion and other 'non-optimized-for-3+core' programs a significant boost, very noticeable in-game. On the other hand, games like GTA4 and the Crysis series run remarkably slower because they are optimized to take advantage of 3+cores.
Remember if you are planning on making a separate profile like this, be sure to test it for stability *THOROUGHLY*. Extensive testing is the only way to be comfortable with a setup like this. 12 hours of prime95, 12 hours memtestx86 (or 64),windows memory diagnostic extended test (at least 2 passes), LINX, OCCT, etc..
Don't fool yourself into thinking that just because you are able to boot into windows and things are running well that your work is done. Test, test, test. And then test a little more just to be safe =P. Catastrophic failure could result if you don't. Just my 2cents.
/M.Walfish