I used my PPL 3.10 to export a 16-bit tiff and use binary editor to trace through the TIFF file. The answer: it is 8-bit stuffed as 16-bit. What's worse is that it is actually correctly stuffed. Say if a pixel has a Red value of 128 (in binary world it is 80 hex) for 8-bit value. The stuffed 16-bit pixel should theoritically be 80 00 hex (or 128 x 256). But the PPL exported 16-bit TIFF pixels by dup the 8-bit as 80 80 hex (or 128x256 + 128). Since most apps will only display 8-bit anyway, the result is the same (lower 8-bit is simply discarded). However if some app tries to be perfect and take 16-bit value as is, it will be in fact the incorrect 16-bit value.

So to conclude, there is no value to save as 16-bit TIFF in PPL. 8-bit TIFF is the same thing and waste less space and less potential to cause processing error due to the incorrect stuffing of PPL.