At default the R5 1600 has a base clock of 3.2GHz for 12 Threads and 3.6GHz ST turbo. The Gaming performance difference should be very close (5-10%).
Overclocking the R5 1600 to 4GHz all 12 threads will make it 50%+ faster in MT than the 8400. Yes it will consume more but it will make it a lot faster too.
5 - 10% in GPU bound scenarios, perhaps. CPU bound benchmarks will show a far greater difference
I would expect the gaming performance of the stock i5 8400 to at least exceed that of the 6700K, since they are both clocked at 4GHz but the i5 has a slight IPC advantage and 2 more cores (yes I'm aware the 6700K has 2 extra threads due to HT but the 2 extra cores will more than compensate for that)
So we are looking at >20% performance difference in non GPU bound gaming.
I would be staggered if MT performance is anywhere close to 50% better on an overclocked 1600, perhaps in Cinebench but in any real world scenario the difference would be far smaller. Even at clockspeed parity, Ryzen has to overcome an IPC deficit. SMT adds, at most, ~30% of a 'real' core.
Assuming a 10% IPC deficit (I think its greater than that, but lets wait for reviews) that would mean, at clockspeed parity, Ryzen 5 @ 4GHz would be at most, ~20% faster in MT throughput compared to a 6C/6T i5 8400.
The math doesn't add up to a 50% performance differential, not even close.