Great story and I might add it helps to have the movement and command that Santana has, (had?). A few years ago ML players voted his CU as the single best pitch in the game.
In regards to location I recall what Tony LaRussa said when someone asked him how to pitch to Wade Boggs. He said: How would I pitch to Boggs? Hard in and soft away, how would I pitch to Jesus Christ? Hard in and soft away. The conventional wisdom is that since you have to get the head of the bat out in front to hit the inside pitch you have less time. For a pitch away the hitter has to let it get deeper, wait longer and the slower pitch adds to the amount of time a hitter has to wait. Throwing soft in can be effective in spots, Pedro Martinez would throw his screwball/CU into right handers and get them hacking in self defense. If you have a pitch that moves away from a lefthanders bat like a slider or cutter, mixing in a CU in can be useful.
That brings up another point regarding the CU. Most pitchers are what I’ll call up and down pitchers or in and out pitchers. A power pitcher is often an up and down pitcher. Think of Nolan Ryan or Roger Clemens. They can throw the 4 seamer up in the zone or throw the big curve ball breaking into the dirt. Sinker ball slider type pitchers are more in and out. Brandon Webb might be an example, sinker moving down and in, slider down and away. These two types of pitchers throw different types of CUs. Clemons’ split looked like his FB down and then dropped out of sight. Webb’s CU might have more of a horizontal displacement. In other words their CUs supported their up and down or in and out plan.