Burst Length is a parameter defined in the USB 3.0 Specification and is not applicable for USB 2.0. This is the reason why changing the burst length to 2,4,6,8 won't work. It should always remain 1.
As far as the throughput is concerned, indeed 30Mbps(~4 MBps) is very less. It should be like 20-30 MBps. But since the USB endpoints are bulk, it will depend on the USB bus condition also, whether there are some ISOC devices connected which will have higher priority and will decrease the Bulk throughput.