Daniel
as you are a SAP employee you do have access to the SAP HANA developers and their information - I can only recommend to reach out to them in cases like this.
Generally there shouldn't be a change in how the parameters work from ABAP or from SQL. As we don't know your code it's hard to pinpoint the actual reason for this different behaviour.
Concerning the parameters in information models: yes, these are always handed over as strings during the model instantiation (when you execute the model).
A simply precaution for too many implicit type casts is to explicitly cast the data type when using the parameter in your model. Then there will be just a single conversion and you shouldn't actually face any performance difference.
Concerning the data type definition for the parameters: these are aimed towards the front-end of the model. When you put in a date, it will show the date picker and accept date as input.
But it will still be handed down to the model execution as a string representation of your input.
That's at least my perspective on things here. As the internals of the calc. engine working are not externally documented, the SAP HANA core developers are the people you want to talk to.
- Lars