I would try and increase the diameter of the prop used. I would go for something around 37mm, I believe the 535 is 35mm in diameter? LOL, it would make sense.
Maybe stick with a 1.4 pitch multiplier? You could always get those cheaper Carbon Fiber props to experiment with. I'd tackle the slippage problem first before anything else.
I'm a real big fan of people who spend the money to make a RTR boat in to something cool/worth looking at and I understand the process can be frustrating. But I don't want to see this boat end up sold or on eBay.
Ryan