Comparing apples to slightly different apples, when Motor Trend put a different GT350 on the dyno the numbers that came back were 467 hp and 374 lb-ft. The track monster GT350R, with the same engine and power rating, managed 471 hp and 376 lb-ft on the same day. There are many possible reasons for the mild horsepower discrepancy and noticeable torque difference, but the most obvious are that Motor Trend tested its cars on a 100-degree day in the California summer using 91 octane gas. Hennessey's test took place more recently in Texas, it probably wasn't anywhere near as hot, and Texas has a ready supply of 93 octane gas.
Hennessey's high-speed run was a more straightforward challenge, taking the GT350 from a standstill to an indicated 170 miles per hour. With a 0-60 mile per hour time of 5.4 seconds they weren't going for all-out times, just a compelling charge to somewhere near the coupe's "more than 175 mph" top speed. Once Hennessey finishes its supercharger package none of these numbers will matter: the Texas tuner predicts output will bound to 808 hp and 657 lb-ft, good for a 0-60 time of 3.3 seconds and the quarter-mile in 10.8. You can watch the dyno run above and the top-speed effort below.