Here is my opinion. There is no magic formula or velocity for success. There is good velocity and not so good velocity. The top college programs look for good pitchers with good velocity, similar to professional scouts. In fact, all levels look for good pitchers with good velocity because they have a higher ceiling… Easier, if needed, to subtract velocity in order to add movement and command, than add velocity and maintain or increase movement and command. However, there have been many successful pitchers who lacked great velocity. Perhaps the biggest secret to pitching is to be different than the other pitchers. However, that difference has to be something that works rather than just being different.
The biggest advantage a left handed pitcher has involves the fact there are simply less of them. There should always be an advantage hitting opposite side pitchers, mostly due to the breaking pitches moving in opposite directions. That is why switch hitters hit left vrs right and right vrs left. However, there is also a good reason why lefties often have more success than RHPs when they seem to have the same velocity and ability.
Switch hitters especially need to be good hitting from the left side. That is because they will be hitting from the left side 75% of the time or 3 out of every four at bats. All hitters tend to hit better against what they see the most often and that is RHPs.
So LH hitters see Right hand pitchers, much more than LHPs. Also RH hitters see RHPs more often than LHPs. This takes away some of the advantages a RHP has over hitters from either side. I believe if there were an equal number of lefties as RHPs, the advantage of the lefty wouldn’t be as great. The way it is the lefty has the advantage of throwing his breaking ball away from a left handed hitter and the advantage of all hitters seeing a LHP only 25% of the time. The RHP has the same advantage with the breaking ball but he has a disadvantage because hitters see RHPs 75% of the time.
This also gives the left handed hitter the biggest advantage against opposite side pitchers. The breaking ball is easier to hit when coming towards you and you see RHPs 75% of the time. This advantage of hitting left handed can be the reason we see so many more left hand hitters as the level gets higher. If you look at a typical high school team, you see a large majority of right hand hitters. (in most cases)! If you look at the very highest levels, (ie. MLB) you see more and more left handed hitters and switch hitters in the line up.
All of this is part of the reason why LHPs don’t need to have the same velocity as RHPs. For the RHP to be successful he actually has to be much better than other RHPs. And there are many more RHPs than LHPs, so he has more people he needs to be better than.
There has to be something (not just velocity by itself in some cases) that makes a RHP harder to hit. More unusual, you could say, unusual velocity or movement! The good lefties really do have a big advantage. Yes, the very best hitters can hit both, but platooning is fairly common at the highest level. You usually see pinch hitters that are left vrs right and right vrs left, taking advantage of that situation. If a good left hand hitter is in a key situation he is likely to see a left hand relief situational pitcher. These one out situational guys are most often lefties making a living rather than RHPs. Once again because the RHPs outnumber the LHPs. There’s almost always a Right Hander in the bullpen.
Also the pitchers who are most successful against opposite side hitters usually have a good change up or good 12-6 CB in addition to a good moving fastball. Sometimes a great cutter (Rivera) because it is so much different than what hitters are used to. There are certain RHPs who dominate RHHs. This is due to things like arm angle at times. What is out of the ordinary, a different look to hitters, can work well no matter what the match up might be.
Here is one way to look at it… Based on equal ability of LHP and RHP
Pitching advantage based on angles and movement
LHP vrs LHH
RHP vrs RHH
Advantage based on repetition
LHP vrs LHH
LHP vrs RHH
Note: 3 of 4 advantage lefty! Obviously there are many variables.
Also people should understand that some of the best velocity guys happen to be LHPs.