Does anyone know if Jiro shidare can be grown in full sun in Decatur, GA area?
Also, what is the best material to augment soil with for best growth?
Ideally, some protection from the hottest/most intense portion of afternoon sun is desirable for any J maple regardless of location but as 'Jiro Shidare' has relatively sturdy green foliage, you can probably get away with full sun with adequate watering. You may experience some scorch in midseason.
The standard convention is NOT to amend individual planting holes - what you dig out goes back in as backfill. This is to avoid soil interface issues which may hamper proper drainage and good root development. Amendments, if necessary, should be applied as a mulch or topdressing. In heavy clay soils, you can plant high in a wide, shallow planting hole, bringing the necessary soil up to level of the top of the rootball in a small mound or berm.
All Japanese maples prefer acidic soils with good drainage - amending does not necessarily encourage better or more rapid growth unless your soils are naturally deficient and only a soil test can accurately determine that. If you must amend, do so over the largest area you can manage, ideally the entire mature root spread.
Couple of observations:
I've found taht planting Japanese maples in raised beds here in the South really produces the best results. Our clay soils are just too heavy for their liking. Don't be shy about it, either. The larger the raised bed, the better.
Also, our "full sun" is much more intense than full sun in more northern or northwest parts of the country. Afternoon shade is definitely preferred for almost all cultivars of Japanese Maple down here.