I do not really have a good comparison but looking at the LST files between to similar programs. I have 2.50 on one machine and 2.6 on another.

The PBP 2.5 code is using 23196 Bytes
The PBP 2.6 code is using 16220 Bytes

Both are with an 18F4550

This is the part I think ARRAY WRITE will be handy for
Code:
' Write to file
    FAT_src[0] = "X"
    FAT_src[1] = "B"
    FAT_src[2] = "C"
    FAT_count = 3
    Gosub FSfwrite