With the car idling, you should be able to easily feel the individual bursts of exhaust at the tail pipe. If it is a smooth flow, you have an obstructed exhaust, which will limit high RPM power.

Also, can you detect a sputter/misfire as you run out of power? Weak spark will manifest itself under full load and/or high RPM. Can be caused by corroded wires, including primary wires to coil, cracked distributor cap, failing plug wires, worn out spark plugs, failing coil (rare). Arcing in the cap or wires will happen even at low RPM if you floor it. Problems at the coil will likely only be noticed at high RPM.

Next item on checklist would be fuel delivery volume and pressure. Start with a fresh fuel filter and then dig deeper if you have to.