I think there may be a little disconnect here, so let me take a step back and describe how our form handling and event-triggered functions work and then we can find a solution for your workflow 
When we get a form submission, we store the data, kick off an asynchronous spam check via akismet, and do whatever your form/site’s-statically-deployed-content says around interacting with the visitor’s browser.
Once we’ve received the results of the spam-check, if they are good (not-spam), we fire off the submission-received even-triggered function as well as any other form notifications you have.
So - if you read between the lines, the event triggered function both runs after the submission and likely after the user has navigated away from your site. So changing the response there won’t have any impact in for your visitor, only what your statically deployed content is coded to do, will happen. The event-triggered function also isn’t run AT ALL if the entry is marked as spam!
Now, can you do dynamic, real time things with functions? Yes! Just not that event-triggered function I’d suggest that you could for instance POST to a function (not the event-triggered function; those are special and are not user-executable in production) - you’ll need to name it something different so it is a “regular function” not even triggered - but THAT function could:
- do real-time calculation
- present results to customer
- present custom HTTP status codes etc to the customer.
Of course at that point you aren’t using our form handling feature, so your function would also have to store the results of the POST.
You could probably come up with a hybrid situation that both:
- runs your custom function to do real-time calculation and return to visitor
- also uses the automatic event-triggered function to do async writes to an external data source (or similar).
Hope that helps but let me know if not!