Thanks for the information, it's more clear for me now.
Indeed, changing ConvertPixelFormat to Mono16 did the trick.
I didn't find GC_AdcBitDepth PV but I found GC_VideoMode PV and I set it to Mode7 (according to the FLIR documentation 12 bits imaging seems available only in this mode?)
At first glance, it works well. I'll do more tests soon as possible.