The Jap PCB of Fire Shark cannot be beaten for bullet speed, as it just gets faster and faster with every loop.As for there being a cap on difficulty through the loops on Toaplan shooters, it does vary. Some end at the end of the 2nd loop like Hellfire and V-5, but the Japanese Fire Shark board seems to increase bullet speed with every loop ad infinitum. My friend ARC conducted an experiment with Fire Shark - he put it on invincible mode and let the game play overnight while he slept. When he came back to it in the morning, the bullets were teeming down the screen like you wouldn't believe
. At least that's what he told me.
Truxton/Tatsujin's 3rd loop onwards had deadly bullet speed too, but you had the fully sped-up ship to accomodate that madness.
Having watched Mills on Raiden Deluxe's 3rd and 4th loops -> now those are some crazy speed bullets teeming down on you, especially as its a slow ship.
Fever SOS is of course fast.