Actually, the Northstar engine has much more capacity for performance. There are custom built Caddy engines pushing out 1000 HP in street applications. You cant do that with a 3800 unless you have more money than brains. In either case, the tranny is the limiting factor in what you can accomplish.
As far as the "equal" performance, I can't answer that, I can only assume that the added power came will an increase in weight that cancelled out the short-term benefits. Or, perhaps, the test driver couldn't run the car through its proper paces.