802.11ac is spread spectrum - it broadcasts multiple overlapping transmissions at the same frequencies. That's the 2x2 and 3x3 MIMO you sometimes read about - the router is transmitting two or three signals simultaneously over the same frequencies, relying in slight timing differences at the two or three antennas to distinguish them. In spread spectrum broadcasts, the interfering simultaneous transmissions are interpreted as noise. And information rate is determined by the signal to noise ratio, not the signal strength. So cranking up signal strength doesn't necessarily improve data transmission rate.
The best visual analogy I've been able to come up with is if you write horizontally and vertically on the same sheet of paper. The letters are distinctive enough for you to tell which ones are horizontal, and which ones are vertical. And you're able to read in both directions even though the letters are overlapping. If you crank up the signal strength - making the horizontal letters thicker - it just degrades your ability to read the vertical letters. And your overall information transmission rate does not improve, or may even become worse (can't read any of the vertical letters).
So as long as you've got a high enough SnR (signal to noise ratio), it doesn't make sense to blast your transmissions at full power. All that does is degrade the bandwidth of the other spread spectrum transmissions. As long as you're getting close to the advertised top speed, I wouldn't worry about it. Theoretical max after error correction should be around 600 Mbps for 2x2 MIMO, in practice I usually see closer to 350-450 Mbps. If you've got a device which supports 3x3 MIMO (most of the Mac WiFi stuff does), you'll see closer to 1 Gbps theoretical max, 600-700 Mbps in practice.
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