If you have met the requirements for shadowing and know you want dentistry 100%, go enjoy life. Having another 50 hours of specialist wont have an effect on your application. Only do it if you are truly interested to learn more about the specialty, rather than do it to improve your app.
You can probably do better stuff. Like honestly go see if you can watch in a dental lab what they do. That would be nice, and you might even be able to practice some stuff you will actually be doing 1st year in dental school - waxing, making impressions, etc.
If you want to shadow a specialist, go shadow an OMFS for like 20 hours or so, and then call it quits. Doubt orthos, endos would let you shadow.
I agree with the two comments above me. Although I have shadowed an endo and a periodontist without a problem and have met Ortho's who've encouraged me to shadow them. Depends on who you know and how you approach them. I appreciate those hours I spent shadowing a specialist because it gave me an intro into what their day to day is like and some things the speciality entails. I realized I will probably never give perio a second thought. Endo however was very interesting and if that was something I'm good at in dental school, I'd consider it.
Give some strong thought into shadowing/volunteering at a dental lab. I think that would stand out more than shadowing a specialist. I would still suggest shadowing a specialist but consider doing it over the summer, not necessarily to improve your app. Just for your own gain.
I shadowed a general dentist for approx. 80 hours. My post-bacc director recommended that I also shadow specialists, so I shadowed an endo and perio for approx 16 hours each. This was brought up in multiple interviews, including a question that asked me to describe a treatment that I observed at the periodontist's office. I am SO GLAD that I took my post-bacc director's advice and shadowed these specialists. I have no plans to specialize...I'm too old. BUT, in my opinion, shadowing specialists proves that you really understand the ins-and-outs of the dental field. Just my personal experience! Hope it helps.
I like the idea of shadowing, not for your application, but to give you a more well rounded view of dentistry and to see different practice models. Also, some schools require you to shadow at more than one dentist office. If you are applying to Georgia, you need more hours, otherwise, 100 is sufficient.