The Mavic specifications are 20dBm plus 6db antenna for 26dBm. The actual measurement I saw in the facebook group was actually 42mW taken with an RP-SMA connector replacing the 6dB antenna and connected directly to a meter. The output with boost mode enabled in DJI Go 4 was 152 or 157mW (can't remember). Technically that's not 1.5W, but .15W. The point is that the boost mode increases output. In comparison, my sunhams "3W" boosters put out 172mW as measured by Jake at MaxxUAV. Perhaps the output power is dynamic and these tests are not done right, but if you simply try it yourself, you can see the difference flying over same terrain. I can fly right over the new Apple campus 2 miles away with boost mode enabled, where with stock I lose connection before I even get there. Lots of wifi interference in that area.
I'm no expert in antenna's, just letting you know about the option to enable this "boost" mode in DJI Go with a configuration file adjustment.