Not sure, but if it is only "a little bit more" to book into Flexi Saver, at least you would get 100% miles on KF vs 50% for the Saver fare. So for me it would be worth paying the premium, but only if it was really only "a LITTLE bit more"...
FWIW, I had a flight HKG-SIN the other day and was invited to mySQupgrade - the fare class then was H, i.e. a Flexi Saver fare.