Yes actually they do suck in comparison to 781's. Back in 2002 I built 2 454's at the same time. Both engines had similar camshafts (SFT) and same compression (10.5). One 454 had the closed chamber heads and the other had '781's . Game plan was to dyno both engines on the same day and he would pick the best one of the two, install it in his Chevelle and sell the other. This was before I had a dyno so we travelled a little ways to a shop that only tested engines and built headers.
The closed chamber head 454 made 560 hp 540 tq and was VERY sensitive to timing changes and coolant temp. It would drop serious HP/TQ above 160F water and it was very prone to detonation. The open chamber '781 headed 454 made 540HP 560TQ and didn't care if the timing was 34, or 40 or 160F or 200F it always made the same power. It also had a much smoother power curve. My customer kept the 540HP 454 and went high 11's @ 118MPH consistently-the other he sold. I can't really describe it but the closed chamber 454 was really finicky with a jagged spiky power curve. This was before averages were displayed at the bottom of the dyno sheets but IIRC the averages were about 20 points in favour of the '781 headed 454. Those closed chamber heads suck if you leave them as is. J.Rob