Is Ryzen a good cpu choice for RPCS3?

2700X has latency issues between CCX module. All SPU cores are mapped to a single CCX due to latency issues on Ryzen.
8700K has TSX, AMD CPUs don't have TSX or any similar Instruction Set.

I don't know if there are any 2700X benchmarks, but 8700K is better.

What are Advanced Synchronization Facility from AMD?

2700X has latency issues between CCX module. All SPU cores are mapped to a single CCX due to latency issues on Ryzen.
8700K has TSX, AMD CPUs don't have TSX or any similar Instruction Set.

I don't know if there are any 2700X benchmarks, but 8700K is better.

What are Advanced Synchronization Facility from AMD?

A decade old paper from AMD proposing an Instruction Set that was never implemented anywhere.

A decade old paper from AMD proposing an Instruction Set that was never implemented anywhere.

Which Core i7 are suitable for RPCS3?

3770K
4770K
5770K
6770K
7700K
8700K 

In the System Requirements Section of the page there are no detailed Informations.

Yes, it's detailed. It recommends a Quad-Core or up from Haswell+ gens.

If you have any of those i7 CPUs you're good - except for the ones you stated that don't even exist, like 6770K and 5770K.
But I'd specially recommend 6th gen or up for TSX Support. AVX2 support is also introduced in Haswell CPUs.

The ultimate recommendation is 8700K though, as it's the ideal CPU for RPCS3 as is.

Yes, it's detailed. It recommends a Quad-Core or up from Haswell+ gens.

If you have any of those i7 CPUs you're good - except for the ones you stated that don't even exist, like 6770K and 5770K.
But I'd specially recommend 6th gen or up for TSX Support. AVX2 support is also introduced in Haswell CPUs.

The ultimate recommendation is 8700K though, as it's the ideal CPU for RPCS3 as is.

OK I have confused it with i7 6700K

I have 7700K

Until AMD catches up in Single threaded performance. Intel is still recommended for any emulation. That is just a simple fact. anything from a 47XX to 87XX are basically the same for emulation. Since STP are so close to each other I haven't really found a reason to upgrade myself.

There's a huge difference from 4790K from 8700K for RPCS3, it's not like we're talking 4th gen to 6th/7th gen.

More 2 cores and 4 threads, you can reach higher clocks in OC while having better IPC, there's TSX on 8700K (while on 4790K you can have it by using an older microcode, but you may still have issues since TSX is bugged in Haswell).

2700X has latency issues between CCX module. All SPU cores are mapped to a single CCX due to latency issues on Ryzen.
8700K has TSX, AMD CPUs don't have TSX or any similar Instruction Set.

I don't know if there are any 2700X benchmarks, but 8700K is better.

Do you have a rough estimate of how much better is the 8700k (like percentage-wise with respect to frame rate/frame time)?

I am also building a new pc for gaming (new, old, and emulated games), and I'm trying to decide between the 2700x and the i7 8700k (or the upcoming i9 9900k). I was looking to play Metal Gear Peace Walker, and, if they (hopefully!) become fully playable, Red Dead Redemption and Resistance 3. I intend to pair the CPU with a 1080 ti. The displays would be a 3440x1440@120Hz monitor and a 4k TV.

Don't waste your money on Ryzen. Intel CPU's are better for gaming.

I am not by no means saying the 8700 and 4790 are the same at all. Though 5 fps isn't worth it enough, for me, to spend $500+ for 5FPS. Since I would have to do a complete system overhaul. CPU, mobo and ram.