>>>with two and even three passengers it just chugs along trying to plane.
Is that you plus two/three?
Anyway as some have hinted... simple fact is a 6hp is too small to shift that many folks on the plane. My experience with a 3m SIB and 9.8hp is very lively with one adult, still planes with two adults or one adult and two children. Go above that and it struggles.
Ideally if you want to carry that many folks you need a 3.2m-3.5m with 15-20hp. If you have to keep the 275 then this is the ideal combination...
For me doelfins are mostly used to try and correct an outboard/craft/weight issue. Get the outfit right and they are not needed just adding drag as said above.