A speckled ambient mask would definitely create more pronounced sparkles. Maybe you could experiment with rewiring your noise patterns to function as a mask to the ambient channel and rely on ambient rather than specular to create the effect of the sparkles. That would be what I would attempt, it would take a bit of fiddling though and not a guaranteed outcome. An image map mask could definitely do it more predictably, but it would be at the mercy of the UVs about the seams.