We don't find the heavy wetter nappies that absorbent for us and definitely wouldn't do more than 4 hours for us even boosted.they are narrow in the crotch so you're limited with what you can boost with.
Don't fns have their own boosters? Jabula are selling them cheapish atm so I'd say you could get away with one or two extra. LO is a fairly heavy wetter and we've used fns heavy wetter fitteds just fine at night, unboosted, (10-12h).
I've gone ahead and ordered some, Debbie has said she can make one of the boosters with wool for really heavy wetters so I'm hoping that will work well. I also have some narrow boosters in my stash that I can add if needed.