1) I don't think Julius Peppers will have a double-digit sack season
2) I don't think the Packers need him to get to 10 sacks for the Peppers signing to be a success.
Let me explain. Julius Peppers is a pass rusher by trade, but the 3-4 defense does not necessarily require pass-rushing linemen to be a successful scheme. That said, pass rush specialists can still be useful, particularly when they're as physically gifted as Julius Peppers.
While it remains to be seen exactly how Peppers will be used, there is a precedence for a down lineman to have success in Dom Capers' 3-4 scheme. Vic Ketchman astutely and accurately points out that Tony Brackens, a similarly gifted pass rusher, had great success working with Dom Capers in a 3-4 defense when Capers coached in Jacksonville. (I will now go slam my head against a wall for saying nice things about Vic Ketchman.)
Still, for his successful year in 1999 (Brackens had 12 sacks that season), Brackens only had 7.5 the next season, Capers' last in Jacksonville. This, along with the fact that only 26 players in NFL history have had 10 or more sacks at age 34 or greater, leads me to believe that Peppers likely won't reach the ten sack plateau. And that's okay. Here's why.
For years, the Packers have needed a pass rusher to take pressure off of Clay Matthews. Relieving pressure doesn't necessarily mean producing sacks, and with today's rise in advanced statistics, we're seeing more and more that creating pressure on opposing quarterbacks is nearly as good as getting all the way home for a sack. That's where Peppers can be successful.
Clay Matthews will still draw double teams, and that's exactly where Julius Peppers comes in. An athletic rusher (even at age 34, like Peppers) could force the opposing offense to rearrange its pass-rush in such a way that Matthews, Mike Neal, Nick Perry, and all the Packers' other pass rushers will be free to roam.
This is what Peppers (and perhaps more so the coaching staff) will need to do to make this free agent move a successful one. If Julius Peppers can still be a match-up problem for other teams, it won't matter how many sacks he actually gets, because he'll still be affecting the game. That's what the Packers have needed for years, and if Peppers can at least be something of a match-up problem, he'll be worth what the Packers are paying.