How much do you weigh? What other board(s) do you have? This will help determine what will work best for you. I don't know much about the Hellcat, so someone else will have to help you with that. However, the V8's and V6's are good freerace/freeride sails.
The question is - is the iS the right board for you? I have an iS 111 and it is fast, but it is also somewhat technical to sail so I don't think it is the best choice for intermediate sailors for fast freeriding. If you like the challenge of mastering a great slalom race board, then the iS is the board. However, if you just want a comfortable "do all" fast board, then you may want to consider the Futura.
The advantage of using "race" sails with slalom boards is that they offer more control as well as a broader wind range when you are pressing the upper limits of your skill/equipment/wind. For most freeriding, race sails aren't necessary, but they can help you achieve greater top speeds.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.