I'm not a physicist, but my understanding is that it is not possible for an information processing system to expand at or arbitrarily close to the speed of light; if we neglect the time taken for other activities such as mining and manufacturing, the most obvious limit is the speed to which you can accelerate colony ships (which have to be massive enough to not be fried by collision with interstellar hydrogen atoms). The studies I've seen suggest that a few percent of lightspeed is doable given moderate assumptions, a few tens of percent doable if we can get closer to ultimate physical limits, 90%+ doable under extreme assumptions, 99%+ not plausible and 99.9%+ flat-out impossible without some kind of unobtainium.

On the question of ontology, I'm a card-carrying neoplatonist, so you've probably heard my position from other people before :-)