Sas,
I've planted out a number of #15's.Some sit there and sulk for a year and other's never miss a beat.
All were planted out after the heat abated in the autumn.The ones that stalled were in premium
soils,heavily amended,5-1-1,etc...The ones that did exceptionally well were the ones that I continually
up potted in a 50/50 mix of my native clay and a local product that consists of rice hulls,horse manure
and compost.When I transplanted them the same amendment was used to create a similar soil structure
4'-6' in diameter,12"-18" deep.They were planted 6"-8" above grade.
I don't believe the local mix is a panacea,what ever is available locally will do.The key is the soil structure
of the pots and the in ground location should be as close as possible.