By the power of truth, I, while living, have conquered the universe.
Once humanity allied with the San 'Shyuum, our technology and galactic presence became a rival to the Forerunners - the dominant race at the time.
From this, we could make an educated guess to say that we were early into Tier 1.
I'd go as far as to say humanity would have become more advanced than the Forerunners given time, but they waged war on two fronts (against the Flood and the Forerunners). Though humanity defeated the Flood after developing a cure, that was essentially a pivot point for the Forerunners to strike against them whilst they were weakened (since the Human-Flood war resulted in many human controlled planets lost).
Ironic, really. The Forerunners devolving humanity instead of becoming allies with them was essentially what made the Forerunners lose against the Flood, before humanity was devolved the Flood cure was destroyed and the Forerunners were alone.