You sure its 2000 kv ? thats a lot of kv for a large gas size boat,most go for much lower kv ,no way that 70 mm prop is going to work well on such a large amount of kv on 8s thats a possible 49 k rpm loaded 59 k rpm unloaded thats going to create masses of heat and burn thing out pretty dam quick especially with a large prop.
Usually with a large boat like that on 8s with a higher rpm youd be best using a much smaller prop in the 60+ mm range to allow the motor to spin at optimum rpm but as i say thats too high a kv rating for a large boat . You should be aiming for rpm range of nearer 30 k on a fast electric but using smallish props.To use that large prop youd be best with lower kv and less rpm to be able to use gas size props.
The most KV id use on a large boat is 1000 kv and even thats not ideal,thats what im using in my 58 Apache but as i say its not ideal ,just what i had lying around lol .You def need to use smaller props to allow the motor to rev out but as i say im worried about your chosen setup .
Heres one of my 50" cats done as twin electric , speeds similar to yours bit faster actually but hes only using small outrunners on 6s each and there 1250 kv i think which is perfect for 6 s using 57 mm props .If i were you id be trying smaller props in the 60+ mm range .Theres a gas size cat the zelos which is a bit smaller she uses 1000 kv motor on 6-8 s and small prop and runs pretty well.Your prop for your setup is too big imo .If i were you id try something around 65-67 mm.Might sound small but when its spinning at high rpm it should do much better than that large prop thats bogging you down.
https://www.youtube.com/watch?v=veLDws6bc24&index=5&list=FLBvMrXd-byRavNCBWuCxqHg