Please consider adding support for allowing System SmartObjects with the ability to support input, and method parameters like the SmartObjects in the Workflow Analytics category.
There are many SmartObjects in the System Category that would be useful to have access to via the OData API in our custom applications.
We have the same issue. We have built a custom worklist and the Smart object that returns a person's worklist now takes 1- 3 minutes to return results. If we had the ability to pass in a RegularFilter[] like in the GetWorkListItems in the Task SmartObject this would solve our performance problem. The problem is it requires Admin permissions on the workflow and in the workflow server rights (which is not acceptable for our clients). The difference in time by using the GetWorklistItems with the FilterParamer is 1-3 minutes and 4 seconds for the GetWorklistItems in the Tasks. To solve this all we would need is a GetClientWorklistItems SmartObject that has a Regular filter parmeter as an input. We were going to use OData but you can't pass parameters into the Exposed System SmartObject.