I have not measured the difference how much it impacts or improves performance, all I say is that i see that games use hyperthreading. Moot point actually beyond that. Since even 2500k is enough to run any game today, and even handle background tasks.
As for the H100 being overkill, well, my 2600k reaches 63c in LinX using the H100, and that is at 4.5Ghz 1.35V. The other three 2600ks I have overclocked using the H100 is reaching the same degrees, and run on about the same voltage. They do get hot. While 63C is not dangerous, it is as high as I would be comfortable running 24/7 365 days a year, which I do with my system.
For me the H100 is not overkill. But that of course depends on your cpu load and computing habits.
About the mindset, I have to say, I am not out to buy the most expensive part. Far from it. When I got the 2600k it was for purchasing price, same price as the retailers get it for, and it was like $20 or so more than the 2500k. Was not much point in not getting it. Especially since I do a lot of cad, circuit board design and video encoding as well. The H100 was a better fit for my case, thats why I got it. It was expensive, but worth it. There are other air and water coolers out there that will do a decent job. It all depends on how far you are willing to push your cpu and how you use it.