Its not a real leap of faith to assume that the the Gwenhyvar had the first prototype Rapier II squadron putting them through their paces (as the few missions we fly in Rapiers is almost definitely not enough to work out all the bugs, there had to be extensive flight testing before Blair gets his hands on one) since we actually run into them (they're there, therefore they must exist

).
Anyway, its probably a moot point, here are all the conversations (excluding Bossmans descriptions) we have about the Gwenhyvar (thanks to Wedge-I'm referencing your site an awful lot lately

).
Halcyon: I’m very curious to hear your report, Maverick. Where is the Johann? Why didn’t you bring it back?
Maverick: It wasn’t the Johann waiting for us at Nav 2, sir. It was the Gwenhyvar.
Halcyon: The Gwenhyvar! I never thought that story was for real. What happened, Blair?
Maverick: It was real, sir, and they tried to kill us. But it’s dusted now.
All right, Blair just got back from a stressful mission (at least it was stressful for me to finally finish that mission

) against an Exeter type ship that fought against him in Kilrathi space after he's been hearing stories from Bossman about a captured Terran destroyer for the past few days. What would you think?
Shotglass: Major! Good to see you. I heard about the Gwenhyvar. I never believed that old story, either. Anyhow, you heard the latest news? . . .
What did he hear about it? This is neither a positive ID of the Gwenhyvar (I never believed it existed, but you found it, therefore it must) nor a negative ID (I never believed that story, the Exeter was the Johann so the Gwenhyvar could still be just a tall tale).
Spirit: Hello, Maverick. I have been sitting here, thinking. My fiancé… what if he has been captured, like those poor souls aboard the Gwenhyvar? Captured, and forced to betray his own people, over and over again? It is a fate too horrible to contemplate. I will tell you now, if I were to be captured… I think I would rather die. Better that than the dishonour and shame of what happened to the Gwenhyvar’s crew.
Another passage that neither confirms or denies the Exeter we ran into was the Gwenyvar, just a general comment about how awful being captured by the Kilrathi (like her fiance was) would be.
Maverick: We managed to reach Nav 1 without too much difficulty, sir. But Kilrathi-piloted Rapiers and some Dralthi were waiting for us. Those Rapiers were probably the last surviving wing from the Johann… it was very tough, sir. I just hope that communication interceptor didn’t break when I was flying that high-speed Immelmann. There was another enemy wing waiting for us at Nav 2. And I don’t think we should use these captured fighters again, sir – the enemy are using some kind of recognition code and they obviously recognised us as Terran pilots.
Okay, finally an positive ID on what the Exeter was. This is from SM2, a few weeks after SM1, when everyone had had time to wind down from Operation Thor's Hammer and analyze what had happened. Blairs or Spirits targetting information was finally analyzed and the Exeter was identified as the Johann. Meaning the Gwenhyvar could still be out there, doomed to patrol the space lanes for an eternity *cue Twilight Zone theme*
C-ya