why is my 96 neon getting bad gas mileage (120-150 miles on full tank) and driving at higher than normal rpm?

The catalytic converter was completely
plugged, so i removed it and ran a straight
pipe. I put new plugs and wires. Installed
new fuel pump and filter. Ran sea foam
through it with premium gas. Replaced oil
pressure sending unit (it was leaking). New
oil and filter. Sprayed some carb cleaner
through the throttle body while it was
running and had someone give it a little gas
so it wouldn't stall out.

I believe the car to have 70k or less miles.
The speedometer/odometer works off and
on and is only at 57k. But the car is pretty
clean under the hood for a 96.

I have to almost stomp the gas to get it
moving sometimes. It accelerates okay
through 1st and 2nd but from 2nd to 3rd the
rpms drop and i have to press the gas pedal
more to speed up. When i do that it jumps
to 3500-4k rpms until it hits the next gear. It
drives at about 2000-2500 rpms when
driving down the road at 35-45. On the
highway it drives from 3k-3500 to maintain
speed.

I know the rpms are higher than normal and
the car seems to be working too hard.

When the rpms jump up high while
accelerating, i can see a cloud of smoke in
my rear view mirror (granted the converter
is removed).

I think its gotta be the map sensor, throttle
body, or injectors. Or possibly the O2
sensors. I just don't know how to pinpoint
the problem. Id replace everything, but ive
already put a lot of money into it, and new
injectors are expensive.

Im going to hook it up to a code scanner in
the morning to check trouble codes. But
prior to all the repairs i did, it was getting a
misfire code and a code saying fuel system
too lean bank 2.