I haven't used the Retributor Gold spray, mostly because I'm not a huge fan of washing sprayed metallics. I find they don't dry quite the way I want them to, unless I'm really careful.
The other issue I have with spraying metallic paints is that covering them isn't the easiest thing in the world, and they add quite a bit of thickness to the model. But on the Sigmarites, this hardly matters, as 85%+ of the model is gold
You definitely want to apply a wash before drybrushing, in order to accentuate the shadows. Drybrushing will leave a texture on the model, and unlike non-metallic paints, you can't glaze it to get reduce that effect (because doing so will dull the metallic). But Drybrushing is MUCH faster than layering metallics.
To give you an idea, this model was painted with a retributor gold basecoat (brushed on) and highlighted using glazes and layering:
This model did NOT use retributor gold (it started out with Balthazar, because I didn't have Retributor yet), but was drybrushed. Because I didn't have the Liberator's Gold yet, either, you can see how the drybrush up to the highlight is more jarring.
The time difference to paint them was huge (drybrushed model hardly took any time at all). Although the stippled effect on the zoomup is quite dramatic, from a distance of a foot or two or more, it looks fantastic.