You keep repeating this to anyone who'll listen but it won't make it true.
Like you, and around the same time, I returned my noisy 7700K iMac to get a quieter one. Although it wasn't the CPU I changed, it was the 3TB fusion drive. I got one with an SSD and have never had any problem with fan noise since. The other thing you never seem to mention is room temperatures. Perhaps these tests that influenced your thinking were in warmer environments ? Typically mine is in the low 20s C. We had a long hot summer though and I still didn't hear my fans.
Nearly a year and a half since release, these iMacs are still the fastest single thread macs in the lineup (despite what you said about the new mini). All things considered, I think they are probably the best bang for buck macs at the moment, unless you have a heavily threaded workload, in which case the pro options are better.
In your post you never once mentioned what you are doing in your workload.
Like I have said earlier in this thread and in other threads, it's pretty easy to test this. Just do so heavy software video encodes. You can note the temperature of the room as well. BTW, in many of my previous posts (in other threads) I have mentioned room temps as well, with my personal tests IIRC around 22C.
Pretty much anyone who has actually done this test with an i7 iMac has noted that the fan ramps up very quickly, regardless of room temperature. (I don't think anyone has done this in a 15C room though.
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Now, this may be irrelevant to you because you don't do those types of workloads in the first place, but as I've suggested earlier, if this type of workload doesn't matter to you, then perhaps an i7 is unnecessary for you in the first place too.
As for the Mac mini and single-core performance, from a theoretical point of view, the i7-7700K and i7-8700 should be in the same ballpark. However, my posts have been mainly about multi-core performance, and that's where fan noise concerns may be the most significant. These chips almost never run pure single-core though.