I have not directly compared GS3 to Gnex but most GS3 I saw are working similarly to my RHD radio wise. I moved from Gnex to Razr HD on verizon late last year. From what I see, RHD is about 6~11dbm better than Gnex most of time. So yeah it's a bit better, but I was not blown away by that as I expected from legendary Motorola reputation on radio. But big difference with RHD is it holds onto signal without interruption when going in and out of weak signal area. And switching between 3G, LTE is almost instant and seamingless.
The cell radio on CDMA Gnex was just POS even by Samsung standard. The main signal issue with CDMA Gnex is from Samsung's own separate LTE and CDMA modem in my opinion. This setup is not good for seamless switch between 3G and LTE and prone to data drops. But phones from GS3 and on are using Qualcomm's multi-mode one chip modem, so radio switching is smooth and seamingless. I don't see any reason why GS4 wouldn't continue on this trend if not better than GS3 significantly.