You don't really gain anything from arcane in terms of arcane effectiveness. A few minor things, but you build into those anyways.

This is the build I will run at 42. https://classic.wowhead.com/talent-c...-0535020010005

This is what I will run at 45. https://classic.wowhead.com/talent-c...53502001000503

With POM, you have a, I can pull well once every 3 minutes build, and if that's what you're doing, your XP/HR is going to be tanked into the ground. Peregrine showed the way, I'm not sure if he ran a shatter build, I believe he did, but he showed that the efficient way to level in your 20's-30's-40's was to just do 6-8 mobs at a time and AE them down.

I'd be absolutely shocked if going POM first, was more efficient than going shatter first, for this kind of aoe grinding.