I do not have any experience with LTO-2, but I have seen a lot of LTO-4 drives fail in a fashion that would cause a sudden 25% drop in tape capacity. Since you appear to also see exactly 25% drop in tape capacity it is possible that your LTO-2 drive is failing in the same way as the LTO-4 drives I have seen.
An LTO-4 drive will write tracks on the tape in alternating directions. A total of 28 tracks in each direction will be written before the tape is full, and each pass from end to end takes approximately 2 minutes.
The drive will read back all the data it has just written and in case the quality is not good enough the firmware will write another copy transparently to the backup software. This will lose some capacity but protect against many cases of data loss.
However as the drive heads wear down it will reach a point where the firmware will always need to write two copies of every block written, but only in one direction.
Careful measurements of the writing speed would show alternation between the nominal 120MB/s in one direction and 60MB/s in the other direction. If you can feed data to the drive fast enough to ensure that it is always the drive which is the bottleneck, then this distinctive pattern in writing speed is the most reliable way to detect a worn down tape head.