I have no back to back tests or anything, but everyone I've talked to that makes power w/ them said to get rid of it...so I did just that on my Indy 426-4 hemi manifold and the 440-3 on my wedge. The old 426-3 manifold I used to have that was ported by Indy for Mike Daurity's KOS car also had the cloverleaf removed.

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