We carry plates for and fly the ILS 29 (3.9 degrees) at Bodrum with no problem at all for A320 and A321. I suspect that it's not so much an issue of what G/S angle the autopilot can handle as a limitation based on the angle at which the aircraft can maintain the speed. After all - too steep and it's going to accelerate! It manages 3.9 degrees no problem!
As has already been pointed out, the max G/S angle for autoland is 3.15 degrees, which is a limitation based on flare geometry according to Airbus.
Cheers all
