I second what williamoak said like a million pages ago. If the investigator replicated the Student of War/Lorewarden's ability in place of Studied Combat it would instantly be better. Unfortunately, this would kill the idea of scaling d6s that both the Alchemist and Rogue have.
On the other hand, just deleting "and once a creature has become the
target of an investigator’s studied combat, he cannot
become the target of the same investigator’s studied
combat for 24 hours." would make it instantly more effective. It makes sense that the Investigator would have to study its opponent every few rounds because the enemy would be adapting as well.
If the above option is still too strong in balancing, give it some sort of knowledge check in keeping with Keen Recollection. This would be along the same lines as a Ranger's Favored Enemy. Those that you're prepared for, you do well against, while the one's you're not, it's a little harder. Dumped all your skills into knowledge? Awesome! Now you get a small bonus on everything (but you still need to make that check).
I like the second option better, as it still introduces a new mechanic, and that its more swingy rather than a flat bonus. I still want to take the first two Kirin Style feats as well as a single level dip into Student of War, but Studied Combat is insanely weak and inconsistent. It deals less d6s than bombs or sneak attack, comes later, and can't even be used more than once on an enemy. Booooo.