It's not so much that it's isn't plausible, but in this case you don't have to "prove" that it did not happen. The onus is on the presenter. In this case, it's the guy writing the article bringing up the folklore. Without any credible proof...again.
The feds and law enforcement bought LOTS of performance cars over the years, along with all the vanilla stuff. They did modifications for extra law enforcement and surveillance equipment to be added, but to enhance performance as described in the article? Never seen it. Never seen evidence of such stuff. So to buy a TR and use it in law enforcement doesn't even flip the radar up. But to go that deep in the speculation without any shred of proof that it happened is nothing but "I wanna believe" stuff.
It's just that there's no proof that the FBI was buying GNX's and modifying them to go 200 mph. Stupid junk like that which has never, ever, been proven. Bring out some irrefutable evidence and change minds. Until then- Wives' tails...or tales...whatev.