Not a good enough test, if you could test your AT&T sim in the Verizon iPhone (Verizon iPhones should have unlocked SIMs) that would be a better test. The idea, place a call on your AT&T iPhone 7 in a spotty area and see if it drops, then place the call with a Verizon iPhone 7 that has your AT&T SIM in it in the same spot you placed the AT&T iPhone call. See if the reception with the Verizon iPhone 7 on AT&T is better and/or if the call quality is better over the Qualcomm modem.
I've actually done that and have the same results, both my 7+ and my wife's non plus 7. I agonized over leaving AT&T and tried every way to get them to work in my home at an acceptable level. I even compared dB levels of each scenario, the reception was similar so I don't believe that was much of a factor. Once again completely unscientific, only the anecdotal experience of a single person.