The 440 I referenced peaked before 6k, so I don’t think I’d expect more cubes with the same heads/cam combo to peak any higher.

Id say if the 6000 peak only occurred on one or two runs, it could be an anomaly during testing, or some heat soak in the manifold cost the motor some upper rpm power.

It certainly won’t hurt anything to test a few springs if you feel like you “need to know”.