I'm unfamiliar with the sidewinder. How does it work extensors?In my opinion, grip strength is hugely beneficial to shooting pistols well. Much has been said already and I don't have much more to contribute about that. Although, I will give you my two cents on overuse injures since that has come up several times now.
I'm an advocate of CoC grippers, however they WILL give you tendonitis if you use them incorrectly. I advise one set to failure with the heaviest gripper you can fully close 8-10 times.
If you use grippers on a regular basis you absolutely have to train your extensor muscles as well. If you don't you will have problems. Only training your crushing grip is like only training your biceps at the expense of your triceps. Not a good idea.
I don't think the expander bands work that well. I've been using a Sidewinder Xtreme #3 and I'm very impressed. It works [I]great[I] for toning your extensors.
In addition to CoC's and a Sidewinder I recommend farmer's walks with heavy kettlebells and wrist curls. Be sure to give your forearms a day to recover between workouts and whatever you do.... do not mindlessly click out high reps with grippers! That's how you get screwed up. Been there, done that.
By the way, I tend to think of friction as free grip strength. Regardless of your crushing strength, a solid thumbs forward grip combined with grip lotion and/or sandpaper will absolutely benefit your shooting.
Grip hard and prosper, my friends.