The CAP bonus is cumulative, wouldn't be any point adding 5%s in later tiers if you already had 5% in the early tier.

My guess is the bonuses only apply to the job you have the job points on. And that you get the Gift Traits automatically if you have the job points already.

Judging by these info dispatches today, I would guess that the reason for the rather underwhelming effort/reward ratio in job points is that it unlocks a sort of JSE ilvl system, with tiered gear Superior 1, Superior 2 etc. with astronomical amounts of xp required to reach the higher tiers of gear. If I'm right, this would be a marked improvement on regular ilvl, for one thing any mule can get lvl 99 and 119 ilvl with relatively minor effort, but if you have the Superior 1,2,3 tiers it means you really put a lot of time and effort into playing this job. It sort of reminds me of the lvl 75 ancient days, when having 75 meant you had a lot of time spent on that job and were at least relatively pro at that job or you wouldn't have got 75 in the first place. Except of course this new system would be far more so.