I doubt you'll see any real gains except in a lab environment, and even then its theoretical throughput numbers, not anything you'll ever see in the wild. Probably have better CPU/RAM specs, but the limitation is going to be client devices. Unless your client devices are 100% 802.11ax (Wi-Fi 6) then your going to have an impossible time noticing any real difference. That is still years away.
If the price point is the same, I would personally probably just go with the MR55 over the MR53. Not sure what the cost difference is right now though.
The other issue you have to watch out for is 802.11ax is new, so new tech = new bugs we've never even ran into before.
I've used a few in production for testing but I've seen nothing noticeable because all the clients are non-802.11ax.
Adding to NolanHerring's thoughts; as we've seen with previous generations of new WiFi technology, the gains will be accrued, over time, as client devices are replaced: new WiFi6 compatible devices will directly see the benefits themselves - and non-WiF6 devices will be rewarded with extra airtime, freed up by the newer devices transferring their data more quickly.
It's also likely that WiFi6 will offer greater capacity. Given that many WiFi solutions are replaced simply to support greater numbers of clients and/or more intensive applications, MR55 should be more scaleable, long-term.
Also remember that buying newer tech always has another potential longevity advantage. MR55 will, almost inevitably, be fully supported, beyond the End of Life of MR52/3. (having said that - remember Meraki supports products for 7 years beyond End of Sale anywau - longer than anyone else in the industry, AFAIK)
I have MR55s installed in two different environments, Cisco 3850-24UX and Meraki MS355. Both switches are provide 5GB to the APs with 20GB to the core. my.meraki.com loads significantly faster but I'm not seeing any health reports coming back into my dashboard. I have clients reporting improved connections and quicker access. My webex sessions appear to be less jittery as well. I would say if it's in the budget MR55s are the progression to go to. I do have MR53s as well that are rock solid. I'd splurge. Also note on the firmware side they run beta currently, so putting them into their own network is highly recommended to stabilize the wifi environment.
We just outfitted our new office with MR55s figuring we'd grow into them. The results have been terrible. After a week of testing and tweaking we have resorted to swapping out the WAPs with the MR52s from our old office. Client connections have stabilized and speed tests have improved by about 4x.