I tried many sets of data and, from what I have seen, precomp should be renamed Uber_Alpha_Proccess...
From 10 sets of data, only 2 were successfully compressed and extracted, 6 times fail at compression and 2 at decompression (CRC error).
It's far too random to be reliable so better not use it...I know some do not have problem with (hard to believe) but it's clearly not my case.
Thx for your time and advises razor