R03 includes regulatory fixes that somehow makes the performance unstable while R01 branch is the "generic" global firmware
R03 has better performance for me. I get much better upload speeds. So I guess it really depends on your tower,location and personal situation doesn't it?
Most probably it's operator-related.
I upgraded to the once suggested RM520NGLAAR03A03M4G_01.201.01.201 and it was really a nightmare (disconnections, band aggregation problems) until I tried RM520NGLAAR03A04M4G_01.202.01.202 and that finally gave me stability again.
Now, out of curiosity, I tried RM520NGLAAR01A08M4G_01.205.01.205 and it's as stable as RM520NGLAAR03A04M4G_01.202.01.202 for me, go figure....
I agree with this. I see a lot of variability between carriers such that there is not a single firmware that runs best in all scenarios. It takes some testing to see what works best for your situation.
As @Max3 has stated, I think it is best to stick with either of the latest firmware visions only.
RM520NGLAAR01A08M4G_01.205.01.205 or RM520NGLAAR03A04M4G_01.202.01.202
They have been updated for a reason. As a side note, TMO has been doing a lot more testing with their 5G SA mode Country wide as of late so this in itself can be a major contributing factor for some users in certain areas.