I am not certain that there aren't different approaches to RTI.
I assume RTI is some software that supports inter alia event
driven simulation with suitable messaging of events to all
or part of "objects" in system.
RTI on Origin2000 or SP2 could support more objects because
it has a network of higher bandwidth and lower latency than any
standard distributed system. In particular on a shared memory system
people dream of better performance in that one can exchange pointers
to shared memory and not send the data between processors
Actually this dream is I think just a dream for classic time
stepped simulations due to nature of SGI memory hierarchy. Sending data
seems as though it takes more time but not so in many cases as the
cost of accessing shared memory is very high. Thus you gain from data locality if you use classic MPI style data copying
For event driven simulations, where not clear which messages are needed
by which objects, situation could be different. Essentially on Origin2000
one needs I assume some optimized shared memory event queue. A field where there is enormous amount of research.

In any case, I don't see why Corba is a necessary step. CORBA is actually
a more interesting increase in functionality but "orthogonal" direction
Rather one should need to "just" replace underlying message transport/event queuing system.