Well you really should be able to do better with such a case, take the time to rewire everything one of those days. But I don't think it'll help much with temps, although that ATX cable crossing over the CPU is not the best thing. Plenty of room to breathe there as you said, but such a build deserves better cable management.
It could be normal for the stock cooler to allow such idle temps, it's nothing dramatic, but with the room temperatures I would expect at least one of your cores to hover somewhere around 25 C, and the "hottest" core to be somewhere around 30-32C. Temps running video benchmark are quite okay for a stock cooler, so the most important part seems to be under limited control - you could easily get better with proper cooling. Meanwhile, perhaps a little too much TIM, perhaps nothing wrong at all with the temps you're getting.
Such a build, yet again, also deserves a proper cooling, IMHO. You should be quit for around 60$ for a high-end air cooler, or entry level closed water loop.