A couple things come to mind reading your message. Full throttle and left rudder are usually a bad combination but you should be able to correct left at that speed. If the boat isn't turning at all left at that speed it's most likely because your servo doesn't have enough torque. 150 oz-in seems pretty good for most stuff but not a gas boat. It takes less torque to turn right because the prop is helping you go right. It's fighting you going left. The reason it spins out when you slow down is because by the time it can finally turn the rudder, it goes full throw, and bang your turned around.
I use a Savox SA1256TG servo for all of my gas stuff (there are others out there that are probably more powerful now but this one works for me). It has 223 oz-in at 4.8 volts and 278 oz-in at 6 volts.
I was thinking about the turn fin on the left also, but in this case I think its a servo issue.
Hope this helps,
Brian
