You don't want a nut extension unless you intend to play lap style or exclusively with
slide. About 70% of my slide playing is fretted, and in blues that's typical. A nut extension will make fretted playing impossible. You don't need it, anyway. I'm doing fine right now with .009-.046 GHSs on my Agile AS-820, stock strings with a stock setup. I've held off on putting heavier strings on, because I'm not sure I want to make this an exclusively slide guitar. That is, I don't want to.