Can I remove power from a bank to conserve power?
No, you should not do this. When power is removed from VCCIO with the core powered, the I/Os still try to drive and read their pins. Transistors are in an unknown state, which results in more current than if the I/Os were disabled (inputs disabled using Data Gate and outputs tristated with termination to prevent floating pins).
When VCCINT is powered, inputs are still "on". Consequently, the data entering the core logic is unknown since the inputs are unreliable with VCCIO unpowered. Outputs attempt to drive as well, with varied results, based on the state of VCCIO. Since this is outside of the operating conditions listed in the data sheet, exact behavior under these conditions is not characterized and not recommended.