As delivered stock - the ilmors put more power to the props than the Mercs for 2 reasons and are much lighter.
Back to the power.
Dustin Whipple himself has dynod multitudinous merc 6 and 700's and according to his dyno they do not make the factory rated power. The Merc 525 consistently makes more like 545 but the blown mercs dont.
Ilmors dont come off the dyno till they rate-its their policy.
Combine this with the Indy drives lower parasitic loss than the NXT's and the Ilmor package wins out of the box.