I have the exact same CPU, as I also wanted to keep the TDP lower. For me it was to build a noiseless HTPC (I have a 150W PicoPSU so I might be able to use a separate low power GPU, but again, I don't want any extra heat sources in the minimal case, which has a passive heatpipe-based cooling). Temps are tolerable, 40-50C idle and 70-80 when playing dolphin, synthetic stress test can take the temp up to 85-90C but this does not cause throttling, or I have not seen such in my testing.
When it comes to Dolphin, the CPU can take it anytime, the Dolphin CPU test gives me a time of 9min 6s (+- a couple of seconds). Turbo Boost rises the frequency from ~3.1GHz to 3.50-3.60GHz. As you already know, the CPU isn't overclockable, which I take as a relief, as I don't need to take the time to find out the sweet spot, I just have to live with the limitations I chose.
The real bottleneck for gaming is the GPU. It CAN handle most games, even modern ones (Bioshock Infinite) with modest settings (especially 1080p makes a BIG hit on FPS, I settle with 720p. Also anti-aliasing makes a big hit usually. I haven't even tested texture filtering
). And now Steam In-home streaming helps me a lot, so I can use my desktop PC to run PC games.
With Dolphin, HD 4600 is enough, if you're willing to settle for 1x IR with more demanding games and to 1,5-2x IR in lighter ones. Even with 1x IR I get GPU-based slowdowns in Super Mario Galaxy 1/2 when useing LLE audio. But as the slowdowns are caused by GPU bottleneck, these don't necessarily cause slowdown of audio and/or game speed, when I use frame skipping = 1. In worst places the setting can cause a skip of >10 frames and I have had some "instant deaths" because of these, but ONLY in some wide area levels. And this is with LLE audio, now that you can use HLE since March 2014 (I haven't switched, yet, since I enjoy the "perfect" audio that LLE provides, HLE still has some balancing / effect flaws, doesn't it?) I was able to rise the IR to 1,5x and most of the slowdowns were gone (it was just a quick test, I should test again some time soon). I don't know about other demanding games (Zelda TP, Xenoblade Chronicles ect) since I don't have them.
Most games will run ok with IR 1,5x, though (and it looks much prettier than Wii already).
So I'm all for Intel HD, and I'd recommend it for any HTPC setup, IF you don't expect to play anything with higher resolutions. As a CPU 4670S is enough for anything nowadays, exept for tasks that require overclockability (=Dolphin... + OC'ing fun + what else?
.
To sum it up (and to answer your question): I'd say all GC/Wii games ARE playable, the most demanding games are (occasionally) NOT fluent, but even then the games are enjoyable.
