Is there a reason you specifically need to know the max transmit power, as it is not always a simple number to give (regulatory limits are different from country to country and other reasons) or also the one you really need to care about in terms of which AP is potentially capable of higher throughput…
A higher transmit power will of course allow the signal to propagate further, and at a greater distance from the AP will also allow a client to potentially have a stronger signal (RSSI) and a better quality signal (SNR) which in turn potentially allows the successful use of a higher data rate over the air (“potentially” is important when you consider the typical capabilities of a mobile device in terms of its TX power and antenna setup compared to an AP), but it is not the main function you need to consider - I can have a device 1m away from an AP with a 3dBm TX power and get pretty good performance after all…
Regarding the “maximum speed” thing…
The numbers you are looking at here are the theoretical maximums based on three key functions:
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Channel bandwidth - i.e. 20/40/80/160MHz.
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Number of spatial streams (SS) on both the AP side and client side (this is the number of TX/RX radio chains aka Mimo available).
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The 802.11 PHY used - i.e. is it an 11n/ac/ax radio, the newer standards allow more complex modulation and therefore higher data rates to be realised.
In modern networks the “data rate” over the air is defined by the MCS rate used by the client and AP to communicate - at a simple level the wider the channel and greater the number of spatial streams in use the higher the potential throughput.
Most vendors like big numbers on their data sheets so they typically quote the “maximum throughput” based on using the widest channel supported by their equipment and with the maximum number of spatial streams possible in use.
If you look at the tables linked below, go as far to the right of them and then as far down as the hardware supports - that is the number the vendor likes to quote!
This is often a bit disingenuous and why as a wireless engineer I spend a large portion of my life dispelling myths about “gigabit wifi” and trying to set realistic expectations for what you may see in the real world when you add a bit of interference and many other external factors into the mix such as client capabilities (the numbers I design against for capacity planning purposes in densely deployed networks are nowhere close to the ones you see on data sheets both in terms of raw throughput and also how many clients you can load onto an AP).
To give you a bit of context, the majority of low end devices are 1SS (1x Tx and 1x Rx radio chain), mid/higher end stuff is typically 2SS and there are things like the Macbook Pro and other high end laptops that are typically 3SS. In the world of 802.11ax (WiFi 6) we have APs sporting up to 8SS - there are other reasons for this besides what the client has but let’s not get into that here as they are myriad and complex 
In the case of the devices you highlight, the AC Mini, 20X and the Soho Mk3 the fundamental difference between them is the number of spatial streams they support.
The Mini and 20X are 2SS, the Mk3 is 3SS (all are 11ac/Wifi5) and that really is all there is to it, the transmit power of the AP has nothing to do with what it is theoretically capable of delivering over the air.
You may see differences quoted for 2.4GHz vs 5GHz because when it comes to saving power and cost of the hardware it is common to install say a 2SS radio for the 2.4GHz side and a higher performing 3SS radio on the 5GHz side.
