I don't think you would gain a great deal, I own the Sigma 85/1.4 and it is my favourite lens on a crop by a mile (mainly because it's the equivelant of 135mm f/2 on a FF).
Sticking to the crop for the moment you would lose around 20% DOF moving from f/1.8 to f/1.4 on the 85mm, that may sound a lot but it isn't. With a subject distance of approx 15ft (4.5m) at f/1.8 your DOF is approx 7.5 inches (19cm). If you open up to f/1.4 your DOF would be approx 6 inches (15cm). To be honest I often have to step down to f/1.6-1.8 to get enough DOF, also f/1.4 on the 85 is a bit soft.
On the 50mm you would lose the same 20% DOF, in the above example the DOF would go from 22 inches (56cm) to 17.5 inches (44.5).
Obviously the closer you get to your subject the shorter your DOF will be and vice versa.
At the same focal lenght on a FF you won't gain anything if you want the same focal distance. Sigma 85 f/1.4 is almost identical in focal distance and DOF to a Canon 135L f/2 on a FF and I'm not aware of any faster 135mm lenses. Sticking to the above example you could put a Sigma 85/1.4 on a FF, to the same magnification you would have to move closer to your subject from 15ft (4.5m) to 9ft (2.7m), compared to it on the crop your Field of View would increase from 14.5 degrees to 23.5 degrees but your DOF would decrease from 6 inches (15cm) on the crop at 15ft, to 2 inches (5cm) on the FF at 9ft.
That was a bit complicated for 5:30am, I'm waiting to go out shooting and I'd better start packing my bags.