Samuel,
If I'm correct the napa additive your talking about is the diatomaceous earth they use to clean up oil spills, it helps stop slugs from attacking your plants if used as a top dressing but stops being helpful when it is saturated with moisture. I think it has some mineral contributions as a soil amendment, probably has some calcium since it's essentially derived from pulverized fossils, but that's more of a hunch for me at this point. I've only recently started adding it to the soil, seems to help.
The grit additives are all very comparable when you get right down to it. I do prefer pumice when I can get it because of its permeable surface which the roots seem to cling to, but essentially scoria, chicken grit, pearlite, and pea gravel are all drainage additives, their density is frankly the biggest difference between them, though each has slightly different properties which have a slight effect on their performance in succulent soil. Any of them will help.
A couple things I feel like you should know: Not all succulents have the same needs, echeveria and most of the subtropical species often used as house plants prefer far less water than sempervivum. They come from dry deserts where moisture is scarce. Sempervivum and ground cover type sedum are from alpine mountain climates, where rain is more common, but drains quickly away from the rocky outcroppings where these plants grow. Ultimately generic succulent/cactus mix is predominantly designed for desert plants, not alpines, though if you water just right it will work for sempervivum to, it's not the best. You'd be better off amending a regular potting soil by adding some inorganic materials like sand and gravel (or whatever you choose to use for drainage from the list above). Miracle Grow Moisture Control has an additive to help with the hydrophobic issue, so that's a good base to start with but any potting soil will work, I use about 4 parts potting soil 1 part gravel, .5 part sand and .5 charcoal. That gives you an easily made mix that works better than the cactus stuff. I hate to discourage you from growing sempervivum, but they do really grow best outdoors also, you can grow them fine inside if you have a good window but they really show their best colors and forms outdoors. Avoid taking them directly from the indoors to an outdoor full sun location though, they need time to develop a "suntan" in the shade or during a season other than summer before they can handle full summer sun.
Regarding Turface, it is an odd manmade material that was designed to aerate grass, and in higher concentrations to prevent the growth of weeds and reduce the mud on baseball fields. You can buy turface at niche sports field maintenance or lawn care stores and 50lbs shouldn't cost you much more than ten dollars, maybe fifteen. Whats odd about turface is that it helps drainage in that it breaks up and loosens the soil, but it also paradoxically holds moisture like a clay would. Not a fantastic material but it could be used to make some improvements. It's best for keeping soil loose, but I haven't really figured it out yet. Used to use it in high concentrations but it added too much air into the soil for plants to stay hydrated. Now I only use it in moderation.
Goodluck,
-Sol
