I don't know about using them to thin, but my trainer used them to actually shorten manes, it left the ends looking wispy and natural not blunt and cut off.
As far as the thinning shears, I've used them on big horses to thin manes with good results. Im a cosmetologist, and one day decided I wanted to see what it would do. Make sure if you do it, you go into the mane at more of a diagonal/vertical, not horizontal, or you will have a mess.
Also, if your just using them on horses, you can get the thinning shears at the "dollar tree" store. The ones cosmetologists are too expensive to be using on horses, and the texture of horse hair dulls them VERY quickly, better to just get the $1 ones and use em a few times, then pitch em and get new ones.