I'm a bit troubled by the claimed figures of the 87 GN line-up. For it's weight and speed it seems like the horsepower would totally exceed 245. Many modern high-end cars that are lighter and with more rated horsepower are still far slower than the GN with 0-60 and about equal or less on the quarter mile. The torque seems about right for 360 ft/lb, though maybe not at 2k RPM. Maybe I'm mislead about horsepower all together but there seems to be a huge discrepancy. This isn't a light car yet can take out lighter cars with more "power". I've also often seen dyno setups used with no air blowing into the engine which could mess up the accuracy. Some have said the horsepower was under rated for insurance reasons...of so it must have been by alot. Any insight? I'm just very curious.
I think its pretty well accepted that the TR HP numbers were a bit underrated.
In addition, GM policy dictates that no other car in the lineup can advertise more HP than the Corvette. Might want to check the HP figures for '86 and '87 Corvettes for the answer to your questions. Though I've seen those shown as 230 and 240 HP, respectively, some poking around reveals that there were apparently aluminum headed motors available that were shown rated as 5 HP higher. Maybe some Corvette enthusiast can clear that up.
The GNX's were dyno'ed on the line, as I recall. Their spec was 276 HP, but I believe actual figures were coming out at or above 300 HP. Note that the GNX was really 'from' ASC McClaren, and thus wasn't limited by the rule above.
Regardless, its years later and the advertising isn't nearly as important as the facts.