In settings/more/about phone look at the baseband version. This tells you which modem you are running. Yes flash modem in recovery. I wipe cache before I reboot, but that is just me.
24 hours later, I am seeing much improved LTE performance. I use "advanced signal strength" app to measure.
May play with the other modems to see if there is a better option, so much of this is phone/network dependent.
Am glad I stuck with the original model. Was pretty peeved initially with the piss poor reception compared to my S3. Figured they would release new modems eventually, overall the rate of SW updates has been pretty low compared to the S3, not sure what that means but it is interesting.