Hello @pep! Welcome to Ext.NET Forums!
What about you used remote filtering, so you can be sure what the user sees is what the server provides? Then it would be easy to know what to filter out of the grid from server-side.
Have a look on this example: Grid Panel - Remote Filtering, Sorting and Paging
You may consider also investigating other grid panel examples in our MVC examples explorer
for other options! Additionally, there are many more examples in the WebForms examples explorer
that may bring good ideas and be easy to port from WebForms to MVC, once you compare a couple matching examples in both MVC and WebForms sample list.
Hope this helps!