AFAIK FoxFi is used to enable tethering on devices where the carrier has disabled it completely. Trying to get unlimited tethering rather than limited to 3GB, is a different issue I think, which isn't what FoxFi is for.
I guess there's something low-level system that tells the carrier, what data packets are used by just the phone, which is unlimited, and what data packets are being used by the hotspot service, and is limited. There's been some discussion about this recently in the Three UK Tethering thread. Thing with Sprint from what I understand, is that Sprint phones are locked so that all data goes through Sprint's proxy servers And so might be impossible to fool their system.