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From reading this the main knock seems to be the stock parts aren't really 1.5 while the rollers are 1.6.

Any reason, or maybe its already available, why not have "true" 1.5 or 1.6 stamped parts?

You can get true 1.5 or 1.6 doctile iron adjustable rockers that are non roller. But there again you will need to get push rods for adjustable rockers with hydraulic lifters.