I'll start with Downie. It would be a mistake. The Penguins have shown that they want to be "gritty". Moving Downie and his more than reasonable 1.0M cap hit would be moving the only "gritty" player they have that actually has a pair of hands to make actual hockey plays.
There are other ways to shed salary and get better. I'll give you one guess...
As for the Penguins and their current situation on defense. This is what they could roll out for the playoffs:
That is fine. And if you really needed to shelter a pairing you could flop Despres and Pouliot and really lean on the top four. Letang, Martin, and Ehrhoff are all capable of playing 25 minutes a night. While it isn't ideal to play guys that much every game in the playoff grind I also don't believe they would have to either.
You drafted Derrick Pouliot 8th overall. For the same reason you want "experience" I would argue "pedigree" should play a role here as well. You can't keep trading assets away for "experience". At some point in time if you are going to win a Stanley Cup you need to get contributions from players on ELC contracts. It happens every single year for the champion. Keep Pouliot in the lineup on a nightly basis and allow him to keep getting better.
And you better damn sure not trade for another defenseman based on how many games the guy has played in the NHL. Judge him by what he actually does on the ice, not the "experience". The Penguins can't afford another Doug Murray fiasco. That is what a Jan Hejda or Scott Hannan type would be.