I assume you mean upper intake gasket is missing?
I would think you would have a stumbling idle if it was leaking air.
It could be a clogged exhaust issue, you "gutted" the cat??, what about muffler condition.
FYI, Federal Law, so in all states, sets a $2,500 fine for "modified" cat converters, and they in no way limit performance if working properly.
I have never ran across an issue with rear O2 sensor causing a performance issue, BUT(big but, lol) that being said I wouldn't put it past the software writers for the computer to add a limiter if CEL(check engine light) was on from a high emissions report from rear O2 sensor.
O2 sensors don't work under 600degF so are not used for at least 5 minutes after cold start, I would reset computer, then I would test if you get better performance in the first few minutes after cold start, if so it could be a software program restricting performance because of the no Cat issue.
But never heard or read about that.
Vacuum gauge is inexpensive and can tell you if exhaust flow is limited.
Also general engine condition
Have a read here on the different tests:
Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine
Do you have a manual or automatic?
Shift solenoids on the automatic could be selecting wrong gear for best torque above a certain MPH.
Older fuel Filter could be limiting full flow, or fuel pump could be wearing out, you could test fuel pressure at 2,500rpms for a minute or two and see if it starts dropping.
And a little FYI about exhaust systems, bigger is not better, velocity is what "lowers" back pressure at the exhaust valves, larger pipes can lower velocity so increase back pressure at the exhaust valves.
That's where the myth of "engines need some back pressure" comes from.
People put on larger header pipes and lost power..... "WTF", engine must need back pressure, lol, correct result wrong conclusion.
The proper pipe size for the engine cylinder volume creates a velocity in the pipe that will create a lower pressure at the other exhaust ports on that bank, this design is above my pay grade but suffice it to say, a properly "tuned" exhaust will create a lower pressure at the exhaust valves.
Which means better performance and better MPG, which is why manufacturers do "tune" exhaust systems for best mid-range performance, after market headers can "retune" them for better low-end or top-end performance, not both, lol, and it just changes the power range Lower in RPMs or Higher in RPMs it doesn't increase it.