There is no simulator available at this time. One of our goals is to have RestAPI backward compatible with previous versions. So, if you have a specific RestAPI call working on the array today - you can assume (I understand you might not want to assume this) that it will work with the future versions also.
Since most of us are forced to resort to using the REST API available with the XtremIO, is there an emulator available that will allow us to test the API at various levels of code? I need to make sure my code is compatible before upgrading. 
XMS / API Emulator