It can have a huge effect. With a poor cell connection or phone searching multiple towers the cellular radio boosts it' transmitting power. It's very difficult to give quantitative results for any trials as so many things can influence comparison tests. Distance, obstructions, amount of competing phone accessing a tower, even having a metal band on your case will reduce the signal strength getting out and force the radio to boost signal.
Qualitatively I can strongly confirm that when I am at home on my strong Wi-Fi signal, I switch off cellular radio because I have AT&T Wi-Fi Connect. My bars go from one or two on cellular to four or five on Wi-Fi. And I get hours more from my battery.
The best test would be to have a dedicated simulated cell tower within a radio isolated faraday cage room to test precisely battery usage. The test consumer reports did on the chip comparisons, incorporated this feature into their tests. I was rather impressed by that as it took reception/transmit variability out of the comparison equation.