1. USB will never give you 480mbps of actual useful data. There is framing, ACKs, retransmits, high level protocol overheads, and such to consider. ~200-300mbps is just about what you'd expect from a USB network connection (depending on packet sizes tested)

2. Most USB controllers do DMA just fine. Maybe you're things of pci-e or firewire-like ability to do a direct write to host ram. That USB lacks (good). But actual DMA for packet data is part of any USB controller nowadays. Even in tiny microcontrollers.