I should qualify my answers.

I have no experience with SPI products. I would listen to them first, or someone who uses their paint daily.

My only experience with epoxy primer is PPG. From everything I have heard, it's used for it's good adhesion, and ability to seal out moisture. If the SPI is formulated in a way to provide those things, ...and can also be used as a "primer/filler", good for them.