niconiconi
(Update: further testing showed a moderate improvement, see thread).Unfortunately, from my quick test, I don't see any network performance improvements in comparison to the 10-year-old ath9k AP. I guess the 300 Mbps IPv4 NAT overhead was not as large as I anticipated, and the RF front-end on my old router was not fried in contrary to my suspicion. It's just plain bad signal reception (but perhaps my tablet's antenna is defective after dropping it one time). I may or may not try tweaking some 802.11 options tomorrow. (Update: further testing showed a moderate improvement, see thread).
(Update: further testing showed a moderate improvement, see thread).Update: Further testings shows a moderate performance gain. When streaming videos from my NAS, the data rate can occasionally burst to 8 MiB/s (50 Mbps+) although the baseline remains at 3 MiB/s (20 Mbps) like the old ath9k 802.11n. I guess the Wi-Fi hardware needs some warn up before it selects a higher data rate. It can now stream the 1080p Ghost in the Shell anime to the tablet used to be barely watchable. 
As I want to rely on 2.4/5 GHz auto-selection for most devices at home, I couldn't test 5 GHz separately using my tablet as no option exists to force-pick an AP from multiple APs with same SSID on iOS. After a Web search, I found the creative workaround: just ban its MAC address at the 2.4 GHz AP. The speed is still inconsistent, as the reception became even worse - with an RSSI of only -79 dBm. The 250 Mbps PHY rate is also not much better than 150 Mbps at 2.4 GHz. But... the baseline improved from 3 MiB/s to 5 MiB/s, with bursts to 8 MiB/s. In the end, 80 MHz is overall a net win in comparison to 20 MHz.
Jeff Sonstein@niconiconi that is hilarious
I love this, so I@niconiconi I usually configure 5GHz APs for 40MHz channel widths, because unless you actually need the extra speed (and you don’t if your main use-case is anything less intensive than copying large files to and from local servers at full speed), the improved signal strength is worth the throughput decrease, and in environments that aren’t ideal (ie anywhere with walls) it’s usually a net win in performance and reliability.