I was a salesman and eventually sales manager for a Buick-GMC Dealer in the 80's thru 2008 and had both Buick GN's as well as GMC Syclones & Typhoons as company cars. The alleged "FBI Chip" was a reality. According to our Buick Service rep, who drag raced his personal GN (did high 11's on street tires before he got serious) and had the reconfigured PROM, all it did was raise the top speed before the boost and timing parameters would throttle back. The limit on the normal civilian GN's was there due to the tire speed rating, which was true of the Syclone / Typhoon also.
I've taken all of them to the limit back then and it just leveled out, nothing dramatic. A Saab Turbo from the same era reacted quite dramatic with its rev limiter; It would slow down so drastically, you'd swear the engine quit.
Bill