Visual evidence in cases like this can't be taken as proof of anything. The FTL shockwave in STVI and the warp 7 ion storm in "The Catwalk" were seen to be moving relatively slowly when they stuck Excelsior and Enterprise head-on. And, the Enterprise should have been at matching warp with V'Ger for it's entire approach to Earth, yet appeared to be plodding into it at impulse speed.
Most damning of all to the theory that it was the Romulan sun going supernova at subwarp speeds is this: If Romulus was destroyed and the mission failed, why did Spock deploy the Red Matter at all? Why did he "have little time" once Romulus was gone? How does Romulus' destruction alone "threaten to destroy the galaxy"? (Unless The Galaxy is the name of Spock's favourite pub on Romulus