Not exactly...600 MHz antennas haven't been in production for very long and manufacturers are still refining their designs. Since T-Mobile wanted to deploy their spectrum as quickly as possible, they ordered what was available when they started the rollout. Now we're feeling the pain that comes with that early tech. And unfortunately, the shared transmit path means 700 performance takes a hit as well.
There are a lot of much higher performing non-600 4x lowband antennas. For example, Verizon has deployed Amphenol HT4C6318x000 antennas (https://amphenol-antennas.com/product/ht4c6318x000/) throughout the greater Seattle area. These antennas get ~18 dBi gain from 696-900 MHz. As a result, there is a tremendous coverage difference between Verizon and T-Mobile setups from the same site - I'll often drop T-Mobile LTE altogether while Verizon is hovering at -110ish or better.
Newer 600 MHz antennas have significantly more robust coverage patterns than the antennas T-Mobile is deploying. For example the Amphenol TWIN6510LU000G-T/TWIN658LU000G-T (https://amphenol-antennas.com/product/twin6510lu000g-t/ and https://amphenol-antennas.com/product/twin658lu000g-t/) get ~17.1dBi and 16.4 dBi gain respectively in the 617-906 MHz range and both support 4xLowband/4xMidband. That being said, I doubt T-Mobile will climb towers to swap out 600 MHz antennas in the near future. As a result, where T-Mobile's density is similar to AT&T and Verizon, as far as raw coverage goes, AT&T and Verizon will have a definite and noticeable edge.
For what ever reason my magicbox stopped working with LTE backhaul after they sent me the gen 3. No I have the Casa pebble. It uses ethernet/wifi for backhaul. I just got it installed yesterday and mms is working great which was hit or miss with wifi calling. Hopefully VOLTE will start working on it soon as WiFi calling does tend to drop out from time to time.