Ah, I forgot that 60p is new to the GH3. Sorry about that.
While I know that everyone says that 24p looks more professional than 30p, I don't really see it myself. I think the problem is that people who shoot 24p tend to use more professional lighting and know what they are doing than those who shoot 30p on the average. I have seen the examples of people shooting both 24p and 30 in the same lighting environment but I never really saw enough difference to matter if I saw a difference. Perhaps I need to watch it on a bigger screen than my 24" PC monitors (1920X1080) to get the full effect.
Now that I can do it myself, I suppose I should give it a try.
Any suggestions as to what to shoot for a decent test?
As for slow motion having a sweet spot at 60%, I really can't comment. I have no idea where you got that idea or how that can be possible. I always thought that different footage looks better at different percentages of slow down. But OK, if you are happy with 60%, so be it. The GH3 has settings for 40%, 48% and 80%. I don't use them, but the camera has them.
I have seen Twixtor in action and I can say without a doubt that it is an amazing tool. Do check it out. Even if I had not seen it for myself, I trust Chris Meyer's opinion.