Are you already using the 'Modify CSS' option in the calibre driver with a kobo_extra.css file on your H2O? If so, I found the easiest way to apply the font-size change is by adding an extra line to the kobo_extra.css file so it automatically gets added on-the-fly during send-to-device:
Code:
body>div#book-columns>div#book-inner {font-size:1.5em}
This works well on my H2O (and GloHD) and I don't need to keep changing the slider when I switch between epub and kepub. Surprisingly the KA1 'just works' without needing to do any kind of font-size adjustment.