I threw thumbers and tomahawks for 12+ years. I injured my shoulder with a thumber a couple years ago and miss that shot dearly, but I have a ton of experience and advice to give.
First and foremost, make sure you warm up your arm before throwing over hand!!! I can't stress this enough. Coming from a baseball background I'm sure you know this but most people don't. I can't tell you how many times I've seen people walk up cold and launch this shot...then turn around and say ouch. Step back while other people are driving as not to distract them, and practice throwing the shot at 50%. I also like to use a towel which provides a little resistance. Get the arm loose and ready to throw the shot. Helps a lot in the long run.
Back to your question about equipment. Overstable discs are the best for distance. The most used discs for overhand by far are Firebirds, Extremes, and Whippet x. Overstable discs do a long drawn out flip while getting distance.
In contrast, Understable discs, like Valks, sidewinders, etc. will do a very fast flip with much less distance. This can be a very valuable shot when needed so don't disregard. You can throw as an up shot or shorter holes with the same amount of power. Also, and this can be a great shot, a baseball thumber down a tunnel. Understable discs will flip quickly and usually give you a huge skip at the end.
I know a lot of thumber/tommy people and no one I can speak of has success with the faster discs. I never did. I tried Xcalibers, Apes, Bosses and had no success. I think it has to do with the grip and the release point. Not to say you can't do it, I've just never seen it. Try a Champion Firebird, they were my favorite. It can take a pounding and goes far.
I averaged 330+ ft. with my thumbers as I had a baseball background also (I also have a very good backhand). Most people called me a cheater or called it a "junk" shot. Remember, in our sport it doesn't matter how you get there.
If you want to have some real fun, go up in elevation where discs are even more overstable. Regularly I'd go up to Lake Tahoe to play tourneys and easily throw 400ft...no joke. People would be blown away. In Norcal, it helps a lot to be able to go over things. Oh, how I miss it....but we adapt.
Disc on!