Not very scientific, but my results were interesting to me...
I had a six pack hood on my car years ago. Taller than the stock A12 scoops, but not real tall. It went a little over a tenth quicker and 1.5 mph faster w/ the carb sealed to the hood. It was trying to rip off the car at 120+ mph though.

I switched to the 5" cowl hood that was NOT sealed to the carb and the performance didn't change one bit. I expected it to slow a little not being sealed up, but it didn't. It gained weight too...the six pack was a lightweight lift-off hood and the cowl is a heavy bolt on.

cuda (2002...).JPGcuda launch at Darlington (2008).jpg
CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax