I found my zap cat ate money even though I aimed to do it all on the cheap. I had a 40hp yamaha on the back - when I finally got top performance it was brutal! But to get it to perform right I had to use a semi cleaver prop. They tolerate ventilation and are stern lifting so do away with lots of nose rise. I'm sure I read on the australian thunder cat forum (great site by the way) that dole fins aren't great on cats? They also had loads of info on how far to raise your motor for best performance. I think with a semi cleaver they suggest prop pretty well level with the hijackers - but keep an eye on the tell tail very easy to be running dry. you would def need a semi cleaver to raise your engine that far though unless you are able to get your standard prop worked to add some extra cup?
25hp probably isn't man enough to make your boat perform as it should. However I don't think that would explain the bogging down you eluded to- what pressure are you running the hijackers at?
Best of luck with it all- they are fun boats but very finicky to get running right