I just bought the 8600k last week to replace my old 2500k that I bought back in 2011. I wanted the 8700k at first but it was nearly $200 more just for hyperthreading (in canadian dollars)
I'm pretty sure that 8350k should be more than enough to run any mame games.
Even my old 2500k at 4.5ghz could run Gradius 4 or Gauntlet at full speed.
I haven't tested much my new cpu so far but it should be 2-3 times faster at the same clock speed. First thing I notice is my old cpu ran at 70celcius while this one runs at 35-40 celcius.... it's a lot cooler.