915 heads

The X heads are definitely faster.
I have sold a couple sets to FAST guys
I have a set of Don Little prepared X heads that run 10.5/10.6 ET
on My Demon @ 3350Lbs. =Stock Cam/Compression/Carb/Intake (500+ HP on Dyno)
A Much more Restrictive Class than you run on engine mods.
The have eight runs on them and cost $5,000+
$3,200.
Stock Good X Heads= $500.
Stock Good 915's =$275.