It's easier to feel in higher gears, but it's measurable in all gears. It's simply a matter of matching area under the torque curve to your gear ratios up to the point where the dwell time of shifting gears begins to eat into your gains. Traction also becomes and issue when your geared excessively short on the low gears, but traction control helps to mitigate this the same way the q/s helps mitigate shift time.
Based on what people are saying above, the final ratio does seem to hit the rev limiter before a top speed is reached, though I suspect that varies with specific conditions. As for taller gearing providing better fuel economy, it's more complex than that. My 800GS had worse economy in 6th gear than it did in 5th at highway speeds.