I've used them... they help a little... the whistle pitch
changes as fuel comes up the vent line. However on a hard to fill boat.. the fuel may come up the vent line often... so it's still hard to tell if you are full or not.... eventually slopping fuel over anyway.
Check your vent tube and see if it's sagging... a low spot that accumulates fuel. also, there are devices made that route
the fuel that comes up the vent back to the tank in a return line. Never used one though.... might work if you are not real full yet.
Eventually I realized that I don't need my tanks
too full... takes forever to use all that fuel anyway!