Thanks for the update, and I am surprised at the difference in the log with the clustered index removal. I would tend to agree that it doesn't necessarily make sense.

I'm wondering in terms of your timing and chunking, could you insert the data differently? Maybe do something to avoid page splitting?

Likely this is a challenging problem, and I'd actually encourage you to start another thread and post some code, see what suggestions people might have for sections to try and change the way it's being load in the proc.