before i get into a long rant, here's what the car has done

Custom TID 2.5" w/ high flow filter (also has nipple for vaccum line)
GReddy Type-S BOV w/ GReddy IC pipe
GReddy Power Extreme 2.5" Cat-Back (custom)
Defi boost gauge
Lower Shores MBC (set to 8psi)
HKS PFC F-Con
HKS FCD

other things to be noted
o² sensor is not connected (which is why im confused...the o² sensor has a lower code number than the boost sensor, but it only shows the boost sensor code)
Catalytic converter is dead

ok, so the car runs great (96K on the clock) but every once and a while (so long as i'm below 4000rpm) and i hit the gas it'll hesitate and stutter...but so long as i'm above 4000rpm it pulls like a freight train, now the power DOES fall off but im sure that's because my exhaust is bottle necked...even still my gas mileage is only about 13mpg even when i'm babying it...now i DO need to replace my spark plugs as they're about 2 yrs old (i only got the car about 3 months ago)...now im aware that when the ECU sees a bad boost sensor it'll think there's either NO boost or LOTS of boost...i'm not sure because my HKS PFC F-Con has its own base fuel / ignition map that i'm guessing is overriding the factory computer's limp mode maps...now despite the HKS unit not seeing an o² sensor signal, i'm sure it's not running good at all, and if the stock boost sensor is bad, the HKS unit's boost sensor might make up for the fact that it's bad? sorry if i'm at all confusing about this...im just confused...

the car runs great, but i feel it could be better if i could eliminate the codes from the ECU before moving on to the next step of modification (DP and MP *cat elimination*) thanks for any help guys