I guess it’s a trade-off, speed vs. responsiveness/latency. I’ve got a 600/600 Mbps contract which turns out as 625/625 Mbps with latency of 6/12/12 ms. I assume this is very good to start with. Tuning CAKE-settings to approximately 600-605 up and down gives 575/575 Mbps and 6/6/6 ms latency. Speed is then above 90% of nominal and latency reduced to half. See some results below. Though, when looking at the corresponding results from Waveform for an Android device over WiFi, I found the download latency reduced from appr. 25-30 to. 12-13, while the upload did not change much at all. Consecutive tests shows that it remains around 15-17 ms during upload. The majority of numbers are still very good though. The conclusion being, as you put it, that the limit on upload might not actually do that much good except from limiting the upload max speed.
Speedtest from Route10, nominal...
Nominal:
Idle Latency:
5.72 ms (jitter: 0.16ms, low: 5.64ms, high: 6.16ms)
Download:
625.72 Mbps (data used: 289.8 MB)
11.68 ms (jitter: 0.72ms, low: 5.84ms, high: 13.34ms)
Upload:
626.66 Mbps (data used: 298.6 MB)
12.09 ms (jitter: 0.83ms, low: 5.80ms, high: 13.49ms)
Packet Loss: 0.0%
Speedtest from Route10, download limit 602 (upload blank)...
Download limit only (602):
Idle Latency:
5.67 ms (jitter: 0.01ms, low: 5.63ms, high: 5.68ms)
Download:
577.21 Mbps (data used: 288.8 MB)
5.90 ms (jitter: 0.40ms, low: 5.69ms, high: 10.65ms)
Upload:
626.58 Mbps (data used: 284.6 MB)
11.93 ms (jitter: 0.82ms, low: 5.68ms, high: 13.48ms)
Packet Loss: 0.0%
Speedtest from Route10, 602/602...
Download and upload limit (602/602):
Idle Latency:
5.75 ms (jitter: 0.17ms, low: 5.67ms, high: 5.89ms)
Download:
571.86 Mbps (data used: 293.7 MB)
5.94 ms (jitter: 0.17ms, low: 5.76ms, high: 7.12ms)
Upload:
574.07 Mbps (data used: 280.1 MB)
5.82 ms (jitter: 0.14ms, low: 5.71ms, high: 7.68ms)
Packet Loss: 0.0%
Edit: While trying out your suggested approach, I found this potential bug, which I reported here:
Edit 2: Seems like this was related to the Hardware acceleration setting for the Route10 WAN port. Toggling the setting, Enabled → Alternate → Disabled → Enabled, seems to provide sensible results again.