I am judging a PPTQ this weekend and I would like to try it the way they did it at Pro Tour by unpacking all the packs and sleeving them in advance. Is it allowed?

If yes, do I need to stamp all the cards too?

Thread closed until we get an Official answer.

Discussions are on-going, so we won't have a final answer for a few days, at least.

Until then, don't try to emulate what you saw at the Pro Tour. Instead, follow the Best Practices, as used at all three of the Shadows Over Innistrad limited Grand Prix events in mid-April, as explained in Toby's blog post:
http://blogs.magicjudges.org/telliott/2016/04/15/general-rules-for-drafting-dfcs-at-professional-rel/

d:^D