1 - You already own a genuine Fender Precision bass. Your MIJ Fender is a genuine Fender.
2 -
Would you notice a difference between a MIJ and MIA bass if you played them side-by-side?
Maybe.
Would that difference be because on is MIJ and one is MIA?
No way of knowing. Every bass is going to be ever so slightly different. Two basses from the same factory could have a slight difference. There's no way to know if any difference between the two basses is because of the country of origin, or any number of other factors. They could just be different because they're different model P basses built to slightly different specs.
Would the difference mean one is better than the other?
Subjectively yes; objectively no. "Better" is a nebulous term for musical instruments. You might like the sound and feel of one. Someone else might prefer the other.
Even if you do notice a difference, and even if in that difference you prefer the MIA, is it worth the extra cost?
Only you can say for yourself if you like it a £1500 more than your current bass. It is 100% a personal preference and personal finance question. Almost no one in the world other than you will notice the difference. You won't get more gigs because of it. It won't magically make recordings significantly better. The only other people who might even notice will be other bass players on TB arguing over which they prefer.
3 - At this point, really, any factory from any country can make a high quality bass. There's nothing magical about the factory in the USA that Mexico or Indonesia or China can't replicate. The issue is usually in that Fender has different factories building different series of instruments, and they're being built to certain price points. There are gems and dogs made in the American factory. There are also gems and dogs made in the Japanese, Mexican, Indonesian, Korean, and Chinese factories. The reputation of how good each factory is would suggest that there are more gems, and less dogs. But also, doesn't mean that every bass from the better reputation factory is a gem, and it doesn't mean that there are no dogs, and most are neither; they're mostly just well made basses.
Some people say MIA is highest quality. Some people say MIJ is highest quality. The truth is obviously more complicated that that.
Worry less about which factory a certain bass came from, and worry more about how well made it is, how well it plays, and what it sounds like in your own hands.
