In my (1.12) pack, I've got Quark, a very common mod, installed, and it includes a recipe that lets slabs be recombined into a "whole block". In combination with the Chopping Block recipes that allow getting ≥3 slabs per plank with a good axe, it essentially allows duplicating planks at the expense of axe durability and hunger.

I can presumably write some ZenScript to fix this manually for the time being—the simplest option is just to disable slab output from the Chopping Block entirely—but it seems like it would be simpler* on the Java end to make the automatic plank-to-slab recipes be capped at 2 slabs per plank if some configuration option is set. (*The catch is that it probably involves either specializing somehow in the wood-compat generation process or adding a conditional to the wood-recipe generator later, neither of which is elegant, but … it's probably still worthwhile.)

Additionally sawmill wood recipes have a hard coded output of 3 for diamond blades

A quick addendum: out of paranoia I double-checked and the (anvil) recipes for stone slabs do not have the same problem.

An addendum for third-party readers of this issue: I eventually wrote some ZenScript to fix the problem, which is public over in the community section of the Pyrotech documentation. It includes compatibility for a bunch of mods, plus I included in a pair of comments the regular expressions that I used to semi-automate their production from Pyrotech's wood-compat configuration file.

I still think that it would be a useful feature to have a configuration option for Pyrotech to limit slab recipes to 2 slabs per plank on its own, but there's at least a stopgap solution now.