Your modifications should work as planned. You may not need a mesh mod, either... try it without modifications to the impeller first, and then if you want finer bubbles work your way up. I did a 'web wheel' mod to the impeller of my RV pump and it produces some fine bubbles indeed, and it is reversible (you just take the fishing line off)...
You could try something like this before you permanently mod the impeller on the Mag 7.