Dolly737's data point shows the effect of spoilers at 135 kt, but it does not give the deck angle (which of course is of more interest with the spoilers down).
As airspeed reduces to headwind the drag contribution from the spoilers (and any lift) reduces with the square of the airspeed; so, to completely assess the effect of spoilers, one has to do an integration and I've been out of school too long to think of doing that.
But I do have to say that I have seen the effect of spoilers at low speed at a glider field. One day somebody landed a Pilatus, forgot the spoilers and came within a few yards of the fence at the end.
So if spoilers make a big difference to the ground roll of a glider between 35 and 5 kt, they must also make a significant contribution on the big iron at low speed.