1991 Nissan 300zx N/A backpressure

[Supercars88]

Good afternoon Z owners, I have a 1991 N/A 300zx. The 300zx has a borla catback system running borla mufflers and resonators. Along with just the stock cats. This is running 2.25in piping. I took the mufflers off as the sound seemed off and I wanted it to be a bit louder. Now the car not only sounds better, but seems to Breathe better just running the borla resonators and stock cats. How far can I go with the car before backpressure becomes a problem hurting the cars engine/ gas mileage? This car is in prestine condition kept mostly stock except for ingen intakes, and I want this car to last as long as possible before a rebuild. (166k, but very clean) I was thinking of possibly taking the cats off and leaving the borla resonator on or taking the resonator off and leaving the cats. It also has an h pipe. The car also has stock manifolds, no headers or anything. Thank you.

[z.Leinbach]

2.5 all the way to the headers should be ok, anything more it will dampen the fuel millage, just depends on what you are looking for really, most turbo cars like a 3" exhaust from turbo back, If I were you id add high flow cats, or test pipes if you like the smell of raw exhaust, personal perferance really