Paul,
I mailed you a version with mods for faster reads/writes. It compiles for Teensy 3.0 but is not tested on Teensy 3.0 since I have not received the replacement for the missing Teensy 3.0 that Robin sent yesterday.
The results for a AVR Mega with 4096 byte writes and reads are promising.
Type any character to start
Free RAM: 2666
Type is FAT16
File size 5MB
Buffer size 4096 bytes
Starting write test. Please wait up to a minute
Write 536.40 KB/sec
Maximum latency: 10336 usec, Minimum Latency: 6908 usec, Avg Latency: 7592 usecStarting read test. Please wait up to a minute
Read 595.04 KB/sec
Maximum latency: 7984 usec, Minimum Latency: 6804 usec, Avg Latency: 6877 usec