First: the database is the heart of Calibre. Adding extra formats to the LIBRARY will only make a tiny difference to the database (metadata.db)
Second: Kobos can show EPUB. So theoretically, you need do nothing.
Third: Using the Kobo Touch driver will convert that EPUB on the fly, to the device, to KEPUB and NOT add it to the Library
And last: You can convert to KEPUB and that will add to the Library, That increases the number of files (and file space occupied) in the Library.
In this day and age of cheap multi-terabyte drives...so what

What is a few gig among friends