I believe it. The US spec TR6 actually made less power than the smaller TR4A engine, even though it had more displacement. And somewhere I've got a TR5 test report from an independant magazine that showed it was seriously quick (for the time and a measly 2.5 liters).
Never measured it, but cam, compression, exhaust header and a few other minor changes (smaller exhaust stems) made a huge difference in a (previous) TR3A motor. It was some serious competition for the Mustangs (until I started breaking driveline parts). And that was with the original carbs, intake, etc.