You can configure the pins to whatever you want without changing the library. The pin assignment used in the configuration files in the download are for backward compatibility with the first version of the library so users don't have to rewire a working system when upgrading software.
But if you want to reconfigure to free up the i2c pins you can make that change – note however that using pin 12 would conflict with SPI. There are no 'free' pins on a standard arduino board.
The glcd library does not use delayMicroseconds. It use the Arduino delay function for millisecond delays and a routine in delay.h for nanosecond delays. Both these functions take the #define for CPU speed into account.
The smaller than expected increase in performance you are seeing when running on 20Mhz is due to the limitation of the GLCD performance. The library checks to see if the controller chip is ready before doing IO to the GLCD controller chip.