Yep your RPM is too low on big pumps like that I believe the recommended sweet spot for RPM is in the 10,000 to 12,000 range
MHZ says 12;000 rpm for its jet80 so if you are making 8400 no load you are probably putting out less than half the available power the pump can manage.
The SSS 56113 with 360 kv would get you there but at 8000 watts I believe it may not be enough power
Talk to Steve at offshore electrics and ask him for a motor recommendation he has a lot of knowledge in the large fast electric boat arena that transfers to jet boards
Keep us updated on your progress .
When you say it’s slow what speed are you doing?