This was done on Stock 4.9.0 Firmware 
. As we’re in an area where there are low-flying military aircraft, I will be complying with the defaults, even if they are a few dBm under the limit.
But disabling the SKU power tables completely is not sensible at all as it runs everything at max and that is not compliant.
Can I change some settings on 4.8.3 to simulate wifi coverage on 4.8.4 and later releases?
I've made the jump to full openwrt 25 now. Whilst it's a bit of a fiddle it's ok once you're used to it, and for what it's worth the WiFi seems to have better range (even without changing any settings from default).
No. Not without taking the drastic steps already posted on this thread of disabling SKU power tables which will make your device run at max power all the time and be likely non-compliant legally, to say nothing of the increased heat output as I mentioned in my last post.
So, I want to experience degraded wifi coverage so I can decide if upgrade is acceptable for me.
So I suggest you backup your existing config settings in LUCI (just in case) and make a note of anything else you have setup, use a Wi-Fi scanning utility (like Airport Utility if you have an iPhone) that will scan your current strength), then upgrade to 4.8.4, whilst maintaining existing settings. That should keep everything as it is, then re-test Wi-Fi strength with the same utility from the same device in the same position you used it before to see if there are any significant differences. Make sure you allow the Wi-Fi scanner to settle down and take the average reading before and after from each waveband. It’s not going to be 100% accurate because of the different factors and network traffic that can influence the readings, but should give you some idea. If you don’t like it, just restore the previous configuration you saved in LUCI.