I agree with your conclusion, but not with your maths. If a 4790K is only 20% faster than a 4690K with a 14% clock speed advantage, that would make hyperthreading worth only 5%.
It's true that you're not going to see a 50% gain in FPS in today's games (or perhaps any in the foreseeable future) with the i7, but in anything that actually uses the extra threads, hyperthreading is worth well over 5%.
See the Devil's Canyon review.
^ If we divide out a 14% clockspeed advantage, hyperthreading (and perhaps the extra cache) is worth around 43%.
Here we can compare apples to apples, with chips clocked the same. Hyperthreading is worth 33%.
Here, hyperthreading and larger cache are worth a whopping 72%.
Even if you're only getting an extra 30% multithreaded performance out of the extra threads, a 14% increase in clocks * 30% = ~50% extra performance for ~50% extra cost.
Whether or not it's relevant to the OP is for him to decide. Today, an i5 is plenty fast for most games, and he probably won't see scaling in most by buying an i7 (in part because generally, games won't use more than 4 threads), but by some metrics an i7 is actually a better value.