You'll need 4 connectors for 4-way joystick + buttons. You'll need (usually) a coin button, a player 1 start button, an exit button, and whatever other buttons you need for Mame. For most consoles you'd need 4 connectors for a 4-way joystick + start & select, and whatever other buttons. You could probably be OK with 8 or 9, but 6 just isn't enough for most anything. Pacman on free play I guess would work... or other 4 way maze games on free play.
If you have a CRT computer monitor, you can just do HDMI - VGA adapter... no need for the VGA666.
If you're really discerning, then maybe lag would an issue. Personally, using any of my keyboard or gamepad encoders from Ultimarc or GGG, I don't notice lag.