Please move to PowerCLI.
I'm not sure where you got that 256 number?
I thought max 4 controllers and max 15 devices per controller, so 60 devices max.
In any case there is nothing to be done in PowerCLI to increase whatever maximum.
Didn't know that.
It's only for PVSCSI HD it seems.
Also not sure if it is supported since V14 or V15.I version
In any case, the current PowerCLI version only supports upto v10.
You will have to revert to calling the API directly.
What code have you been using in your tests?
Can you share?