I'm having pretty server issues with my s2000. It started with a misfire and hesitation that would go away once warm. It still does this but it now back fires through the intake occasionally when revving. It is very slow to rev but once warm that's the only symptom other than occasionally breaking up.
I've replaced:
Plugs, coils, MAP, valves adjusted, fuel pump, fuel filter, fuel regulator, air filter, passed leak down, passed compression test, clean throttle body, cleaned IACV.
I'm at a loss with it entirely. At this point I'm thinking a bad valve somewhere.

 

Maybe a bad injector.

 

No mods or recent fixes to the cams or ECU?

Maybe, do a leakdown test?

 

Only mods are catback exhaust and high flow cat

 

He did the leakdown but it's good to revisit all variables in case one was missed. This sounds ignition timing related

 

How did it run before those mods? Since you said it backfires and is slow to respond to throttle I'm guessing ignition timing is off.
Like Slow said it could also be a bad injector sticking when it should be opening and closing smoothly. Maybe more than one. Got access to a spare set of OEM ones to test?