2003 1.8 Sentra won’t rev past 5k

[Devbo7344]

I just picked up a Sentra off my sisters friend for $500 to use as a beater and save some money on gas since my truck gets abysmal mpg. It had been sitting for a few years since she got a new car.
It’s an automatic with the 1.8 and has 148k miles.
It runs fine around town, but really struggles to maintain 75 on the highway. When I’m bringing it up to speed around 40 mph I’ll have to be full throttle and it’ll climb to around 5k rpm and then just hang there, if I let off the gas and let it shift it’ll drop to around 2.5k rpm and then slowly make it up to 75.
It won’t go past like 4.8k rpm in any gear. I started from a dead stop and floored it, It’ll speed up until it gets to almost 5k and then just hang there like it’s hitting fuel cut or something. As soon as I let off the gas it’ll upshift and then do the same thing in the next gear.
I really don’t notice it or mind unless I’m trying to do interstate driving. If I’m doing like 70 and encounter even a small hill, it’ll start to loose speed. If I give it gas, it’ll downshift to 4-5k and just hit the fuel cut symptoms, and if I let off it’ll upshift and not be able to maintain speed. Before I know it I’m going like 50 in a 75 lol. On flat ground it drives fine and will cruise 75 all day.
I’m leaning towards fuel pump or filter. I do have a P0420 code, and after doing some googling people seem to talk about the precat a lot. I pulled it off and it’s already been gutted so that’s not the problem. I also cleaned the Maf. I figured I’d ask you guys before I start throwing money at parts. Thanks in advance.

[VStar650CL]

On an '03 you'll have a rear cat located at the back of the flex pipe. Since the main cat is gutted, make sure they dropped the flex pipe and cleaned out any debris that may have traveled aft from the main cat. The rear cat can also melt or disintegrate, just like a front one.

[Devbo7344]

Drove it without the exhaust and it didn’t change anything besides being extra loud lol. Changed the knock, cam and crank position sensors and the fuel pump. Still hits a wall at 4k rpm and won’t rev past it in any gear. I’ve put about 400 miles on it and the P0420 for the gutted pre cat is still the only code. Spark plugs are also new. I’m not sure what else it could be. Maybe injectors?

[VStar650CL]

Injectors won't usually give you a "brick wall" effect at a predictable RPM unless somebody installed the wrong ones. Stream the MAF and make sure the readings make sense, and check the accordion tube connecting the airbox and throttle body for hidden cracks. The MAF should read a bit more than the engine displacement in g/s at idle, probably in the neighborhood of 2.0~2.5 g/s for a QG18, and it should go up smoothly with RPM.