Where should I discuss SD/MMC FAT issues?

[skimask]

We'll, I've done my best and yes, about 84K Tcy is as fast as this thing is going to read 512 bytes from the MMC and write the 512 bytes to the FRAM. It's 83,968 Tcy to be exact. Last night I thought I'd give it one more go.

Add
DEFINE NO_CLRWDT 1 'no extra clear watchdog timer instructions
to the top of your program.
Will probably save a few more cycles there.

Also, in your 'big' loop, this might help somewhat. Remove the 2 for/next loops, replace with this:
loop1:
x1 = 2 : x = 0
loop2:
gosub get_byte : gosub write_i2c : x = x - 1 : if x = 0 then loop2
x1 = x1 - 1 : if x1 = 0 then loop1

Rewriting the 2 gosubs as inline code would save you about 2K cycles at most, might get you down below 20ms, but would use up code space. No real savings there.
Also, go back and try to BANK0 as much as you can, the commonly used variables first, until something doesn't end up in BANK0 (check the .lst file).