This is the closest thing I have found so far comparing performance on M1/16 v M1/8 on iPad Pro.
https://www.youtube.com/watch?v=9bA4jc4J9NQ
Unfortunately he’s using iPadOS 15 beta, its still only June 2021, and apps would not yet take advantage of the higher RAM limits (even assuming it was true of the beta at that time). And even some of the other benchmarks should be re-done with a shipping version of iPadOS 15.