Welcome.
I couldn't find EU test results for MPG (which would have been close enough to EPA as makes no odds) - I don't think the EU tests MPG either EDIT for motorcycles.
Fuelly has a load of entries for VFR800 owners
here which range from very low to very high - those are generally real world users so a good range of experience to compare against or aim for.
As for HHO it sits like a neglected toy in the Unicorn Corral awaiting someone to provide concrete evidence of it doing anything really.
Bennelson has videos on how to do an EV conversion if that helps.