Yes, it makes no sense for Spock to stop (or reverse, or whatever) something that big by dropping a vial into it at Romulus.
We see the speed at which the Red Matter eats away at the shining wavefront, just like we see the speed at which the wavefront hit Romulus. And with the Red Matter effect, we can compare directly to the motion of the Jellyfish, seen delivering the vial and spinning at a supposedly steady rate while at it.
That the effect would negate a wavefront 2 AU across is within the limits of plausible. If the wavefront is 25 ly across, though... No way.
Nevertheless, one has to wonder what was supposed to happen in the ideal case. Letting the star blow and then negating the wavefront would save planets in the "Hobus scenario". It would help little in the more realistic "Homesun scenario", though. Perhaps Red Matter had several beneficial qualities, and Spock's original plan was to use it to stabilize the star before it blew? Or to turn it into a giant black hole, giving Romulus a few days to evacuate rather than a few minutes?
In any case, all it takes to blow the ideal model out of water is a single innocent utterance of "Hobus" in PIC. I sincerely hope it never happens.
Timo Saloniemi