Opinion on weather or not to have the slab on grade foundation engineered. My county does not require this. Any input would be great. The framer I will be using has told me it might be a good idea to get the soil tested by a grotechnical company and the slab designed by an engineer. I thought your slab on grade design is good.
Please see this article if you are in a high wind area:
http://www.countryplans.com/tornado.html
For slab on grade this may be a place to go further than code minimums. There are some scary photos in the aftermath of wind storms.
If you will be subbing out the foundation, make sure the plumbing layout is finalized, and get the sub's assessment of the soils - compression tests are valuable but expensive.