Greetings!

The hardware specs on the mikrotik website says:
hap ax2: Antenna gain dBi for 2.4 GHz 4
hap ax3: Antenna gain dBi for 2.4 GHz 3.3

Does this mean that 2.4 wifi coverage in the hap ax2 is better?

Country limits permitting, hAP ax3 has 2dB higher Tx power for most rates (except lower ones) so it'll offer slightly better throughputs at medium to low signal strengths.

From my findings you get absolutely no benefit on hap ax3 over ax2 due regulations(hap ax3 drops power down compared to ax2 to match the gain from antennas) in most of the world ,except if you live in America or ignore regulatory, 100% same signal/coverage, speeds.

AX3 has much more CPU power...

AX3 has much more CPU power...

How does this relate to coverage by 2.4GHz radio (which was the question in opening post)?

From my findings you get absolutely no benefit on hap ax3 over ax2 due regulations(hap ax3 drops power down compared to ax2 to match the gain from antennas) in most of the world ,except if you live in America or ignore regulatory, 100% same signal/coverage, speeds.

My user experience using both AX2 and AX3 in a house with THICK concrete walls and floorings:
AX3 wins for range and stability of connection.

From my findings you get absolutely no benefit on hap ax3 over ax2 due regulations(hap ax3 drops power down compared to ax2 to match the gain from antennas) in most of the world ,except if you live in America or ignore regulatory, 100% same signal/coverage, speeds.

My user experience using both AX2 and AX3 in a house with THICK concrete walls and floorings:
AX3 wins for range and stability of connection.

It depends on country, whatever you say may be correct for you, but not your next country neighbour...

For example 100% same config:
HAP AX2 runs 5180/ax/Ceee, Tx power 18
HAP AX3 runs 5180/ax/Ceee, Tx power 17

HAP AX2 antena gain 4.5
HAP AX3 antena gain 5.5

So extra 1db gain you get from external antenna on AX3 gets cut down by 1db TX power setting so you end up in EXACTLY the same output in dBi in order to comply with regulations.

The EiRP is one thing the sensitivity is an other! When a device has an antenna with higher gain than it can receive the same client higher signal level than another device with less antenna gain. The EiRP will be the same on both devices. So the AX3 will be better than AX2.

Exactly.
Because of those big ears AX3 has better reception sensitivity and allows for a more stable connection given same distance, same client, same infrastructure in between.

I didn't need any power measurements to see it happening.
I did the tests with both devices on the EXACT same place in my former house (sold now).

Exactly.
Because of those big ears AX3 has better reception sensitivity and allows for a more stable connection given same distance, same client, same infrastructure in between.

I didn't need any power measurements to see it happening.
I did the tests with both devices on the EXACT same place in my former house (sold now).

Those big ears also can recieve interference better if you live in noisy environment and you end up with same thing, you can look at it both ways...

I was testing with iperf towards laptop and mobile phone at fixed point and distance, i just turned one and other router on same spot on desk, 100% same config.

I got within 99% exactly the same speed (iperf server on local server)

But once i changed country to United States and did same test, there was big difference between HAP AX2 and HAP AX3. and i got around 20+% more on AX3.

So i repeat, it depends on your country and thats why some users are happy with this routers and it covers their entire house while some are miserable because regulatory kills output by 1/3...which is HUGE difference.