First, if you haven't already done so, stop using straps, Period! That means don't use them for any exercise, pull-ups, deadlifts, any exercise. All they do is rob you of developing the grip strength to perform the exercise.
Second, realize that there are different types of grip strength (as was already stated). There is support grip strength (which is the kind needed for things like heavy Deadlifts), crushing grip strength (COC grippers), and pinch grip strength (rafter pull-ups) to name a few.
As was also already stated, one type of grip strength doesn't necessarily mean that you will be strong in the other types. I've seen guys who are world champion deadlifters (who could deadlift over 800 lbs) not be able to close even a #2 COC gripper (which takes only about 195 lbs to close).
So, realize that nothing will give you the specific type of grip strength more effectively than actually doing the exercise itself. If you want a strong grip for Deadlifts, keep on deadlifting.
That said, I don't think it would hurt to do other gripping exercises as well. Just try to use exercises that will improve the type of grip strength that you are trying to improve.
Good luck and good training,