Yes, true, the methods have broad phase and partially narrow phase in common, and since the rigid body implementation most likely is a relaxational method, they also have the pairwise iterative term in common (though they look different in the two different models of course, and SPH typically just computes a single pairwise instance for the force).
My experience is though, that it is easier to simulate 10.000 rigid bodies in real-time than 10.000 sph particles in real-time.
A fluid must be fairly incompressible, which is terribly expensive on the timestep, but it must still have dynamics gouverned by the SPH-many-body potential that _stores_ energy (i.e. it can't be hardcore all the way).
The rigid body pair potential need only be rigid, and doesn't need to store potential energy beyond the constraint stabilization, and this is not so demanding on the step size.
Erwin Coumans wrote:His GPU method is not just particles, the rigid bodies allow for stacking:
Harada-san is visiting GDC and we are meeting up again. Hope he will disclose/publish more of his approach soon.
Enjoy,
Erwin