Don't get used to it though, since in patch 2.4.3, weapon swapping will interrput your current cast (to prevent swapping the insane arena haste weapons with regular dmg / heal weapons and benefint both)
I think you need to put items in your "High Mana" and "Low Mana" sets, even if it's just duplicates of your main casting set.
I've just tried that and it's still not working sadly, thanks for the effort though!
Edit - Infact it worked! Thank you, now not sure if the % settings on the various bars (Cast to high/Low to Cast) etc should be set at anything in particular or if it won't affect it as I'm runnind the mod purely for spellsurge swap.
It's definetely not swapping to the spellsurge weapon nearly as often as it should.