You may want to post this in the Container Forum, where this type of container mix is discussed constantly. Since you are using 'Al's Mix' you should address your questions and concerns to Al.

Some of the commercial seed starting mixes are supplemented with fertilizer. In addition, they are presumably optimized for getting seedlings off to a quick start. It's easy for me to imagine that a tiny new root system would do better in something like the commercial peat based mixes than in something as coarse as the gritty mix....it seems like a larger %age of the root would not be in physical contact with a surface of one of the pieces, but rather spanning the spaces between. I get the impression that Al's gritty mix really shines in long term performance.