The cat has to go faster than that or they would never catch the kiters.
You're in the middle of the ocean, and can go 20 knots, and your chase boat can only go 8 knots. Explain to me how you are going to leave the chase boat behind again? It's certainly possible for the cat to only go 6-10 knots. The last 24 hours they have gone 212 miles which I believe is average 8 knots. I believe wind speed is 15 knots.
You can get a faster cat, like one of them racing 60's like an ORMA they are probably 3 times quicker than this charter boat, but you would need to bring a racing crew with it and I don't think they are available to charter, or you could bring a ocean capable speed boat but then you would blow your entire budget on gasoline.
I think the organizers knew this would happen that the chase boat would be pretty slow, but I'm not sure all the kiters understood what they signed up for.