The 9-pin SanDisk cards just implement the standard SDIO bus. SDIO has Vdd, Vss1, Vss2, CLK, CMD, and four data lines.
Modern SD cards have latency no mater what you do. They have internal ram buffers and their internal proprietary algorithms decide when to program flash. You can send 512 bytes then the card may go busy and block you from sending more data.
This is true no matter which bus you use. The spec allows SD/SDHC cards to go busy for up to 250 milliseconds. Amazingly, SDXC cards can go busy for 500 milliseconds.
Few cards have latencies this long but all cards have occasional long latencies for wear leveling and other house keeping.
The class 10 rating assumes an average rate for many MB of data so you must have a ton of buffer to avoid problems with stuff like video.
In short you can't beat this with any of your clever tricks.