I don't think Devils will change something with defense. I don't see it. Siegs and Kovy signed cheap deals, they were top defensive pair in the league. Like is better even not very consistent but much higher in his lows and way much better in his highs. Dougie is dougie, I don't know will he return or not. Peace was very good. Dillion was good enough for his physical/pk role. With Seamus Casey in the mix, with a couple of interesting young LD in Vilen and Edwards who are nice options as 9-10th D. May be they will give some time for recover for Kovy or for Dillion and will sign Dumolin who was great in the play off. Its a huge mass of talented defenseman, a lot of teams have much worse defense and play play off games.
Offense is a different animal. Gritsyuk will definitely help, but as you said it will take time for Jack to return to his form. And the depth isn't good enough at all. Haula and Palat will be one year older. So Devils needs at least one more good offensive forward besides Gritsyuk to create better structure.
Will see how it will go.