No. Back2Back the APs can receive and send at the same channel at the same time. So you double capacity of the tower. A sending AP do not listen while sending. So it is not disturbed by the second AP sending at the same channel. CPEs do only receive the signal of one of the APs. And when front2back of the AP Antennas is good each AP does only see his CPEs..
I stand very very slightly corrected and only in very specific scenarios. If you have a 130Mbps (for instance) modulation only 2 access points in the same channel, and they are both operating with equal send and recieve throughputs, then you can send 130Mbps from the tower making the pair of APs full duplex-ish.
There is absolutely no point in GPS syncing back to back radios. Front to back isolation is a vastly superior method because you can completely re-use the channel. If you GPS sync, you will reduce throughput on both sides wastefully. ONLY GPS sync same direction/overlapping APs.
But, that's not the real world. Most all data off tower APs is going to be async ratio like 70/30 tx/rx. Because of this, you can only 'gain' 30% (or rather gain tx equal to the rx amount on the channel), and this improvement plummets with a 3rd or 4th AP because you only have 100% of the 130Mbps to work with among all APs in channel anyway.
Don't get me wrong, there are gains for GPS sync:
improved compute capabilities (more cpus), port speeds (if stuck at 100Mbps like a rocket M5), and ability to run wider channels so high speed plans are possible. Anyone that uses ubnt rocket M5s will probably agree that about 30 active clients is an ideal peak. If the M5-GPS worked, you could have 60 active clients with 2APs, or 90 with 3APs. Maybe 30% gain in overall throughput because you make a group of APs more 'full duplex' like, but the real gain is more active clients. This is worked around today w/ 10Mhz channels, which is ok but leaves less throughput on the AP so speed plans have to be adjusted down accordingly and channels can't be shoved right up against each other so there is some spectrum waste too.
I would much rather see on-the-wire sync with a dedicated port on the device to sync radios. GPS is added expense and we have the technology to make a high precision clock for a reasonable price now.