If you reduce cabling, you might get close to 14Mhz. But with that resistors, you'll not move further then that.
I've used the sdcard in my tft also, and I've managed to get up to 28Mhz by removing the resistors and just put a shunt in their place.
The only way I've got 42Mhz, was with one sdcard reader like that with wires directly attached to the sdcard reader socket bypassing all the electronics there. Those electronics are for voltage translation and are very slow. Since Due is already 3.3V, you can safely bypass it.
I can place a picture here later for your reference.