I've been using the CoC grippers for years. There is so much to say about these fine tools for grip training that I don't know where to even begin. The CoC grippers will build certain areas of grip strength (crushing) while other ahtletes such as martial artists would need to work on tools that strengthens the fingers especially the thumbs.
I'm sure you've been to the Ironmind website (www.ironmind.com) that carries all of these grip tools. I've got most of them and they are all great.
If you can deadlift 400 then I'd say you could probably handle the #2 gripper. Most gym rats can close a #1 but sometimes they can't. I have a friend who is a cowboy and does rodeos. He also has a ranch and is a truck driver. I gave him the CoC Trainer and he couldn't close it. I was shocked.
A lot of guys that have great grip strength are ones that have never touched a weight in their life and I thought this would be him but it wasn't so you never know.
If you train with the CoC grippers properly you can build great grip strength. One advice I would give is to not do very many reps with these. If you can rep out 12 closes on anyone gripper then buy the next one up. It's also a good idea to have the next highest one that you can't close so you can do negatives.
There is a grip program that was put out a while back called the KTA program. This program is very good to help one achieve closing the #3 for certification. The training is brutal and would work contradictory to your other training.
I've always done competitive powerlifting and used the grippers for grip strength. It's also fun and challenging. My brother-in-law doesn't powerlift and only does grip training exclusively. It's fun to watch him tear a deck of cards in half and then in quarters along with other grip feats.
There is a whole group of athletes that are just grip specialits. Anyway, I never set aside powerlifting to focus on getting the #3 certification. Just keep in mind that if you decide to do the KTA program that your other lifting will probably suffer. I never have had a grip problem when it came to deadlifting. If you are not using straps for most of your training then you are already building the grip strength needed to hold the bar.
There is another company that makes grippers that fall in between the grip strength ratings of the CoC gripper that make it easy to move up to the next gripper in the CoC arsenal. You will find that there is a big difference between the #2 and the #3 gripper by Ironmind. The Beef Builder makes grippers that fall in between those such as a 2.5.
As I said earlier, there is a lot to say about grip training and the use of the CoC grippers. The best resource I know of online that covers this speciality is:
You'll need to sign up as a member to get in there just like here on T-Nation. It's not a big deal and will be worth your time to check it out.