A lot of it will come down to heat. If you were to push a SST-50 to 5A in a P60 module, for example, it would be spectacularly bright for about a minute... until it caught fire.
In a small host, under-driving it at 2.8A gives similar brightness to a P7, a smoother beam than a P7 and maybe even less heat than a P7 at 2.8A (i'm theorising on the less heat bit, feel free to correct me on this)
If you want a balls-out 5A SST-50, then you'll need something like a Mag with a britelumens heatsink, or something the size and heft of a Jetbeam RRT-3, purely for heatsinking reasons.
On a side note: mAh is a measurement of battery capacity. mA is a measurement of current. I think in your OP you mean 2800mA (2.8A).