WAG around 440 I think, sure I'll be corrected before long. Depends on the definition of limit, I'm sure if you strapped a 4x4 manifold to a compltely mental engine it would make more power, the correct term is probably what power level does it become worthwhile and cost effective to change it.
correct you are,
just having issues finding a manifold up to the job that will clear my mounts looks like modify one or get one made!!!!