To answer both questions:
(1) What is the cap on FS XP? The cap on XP is high enough that you can accumulate all the XP you would need in advance, and then train the six FS trees of your choice (assuming you had done quests to unlock 6).
This strategy is actually advisable if you're simply trying to grind to jedi initiate. The skill points you save by NOT training Force Sensative skills can be better used in traditional professions in order to allow you to make XP faster. If you train a FS skill box every time you qualify, you're going to be 23 skill points short when you're going for that last box. Save the skill points, put them into another profession that will get you XP for conversion. When you have all the XP you need for 6 trees, then train it all at one time.
If your primary goal is not Jedi Initiate, and you're more interested in taking advantage of the benefits FS skills might offer your non-jedi character, thenyou might prefer to train in the appropriate FS skills when they become available.
(2) Is there any benefit to getting excess FS XP? Yes, in some specific cases.
After you unlock initiate you have to maintain at least 6 trees of FS skills. If you were not able to unlock a tree that you wanted, but trained in a different tree in order to simply get started on Jedi, then you might want to come back and pick up that preferred tree later.
For example, if you got your XP for 6 trees but you never unlockedthe Ranged Defense column from Heightened Senses then you might want to go ahead and take a different tree that you find overall less useful (Wilderness Survivalfor example). The next cycle of thevillage whenthe Ranged Defense quest comes around you can unlock it and, if you have XP in excess of the 6 trees, you can train in Ranged Defense. Now you would have 7 FS trees, so you could drop the less usefull tree in order to recover 4 Skill points.
Best of luck and have fun in the game.
Oteis / Goteis/ Prolis