Yes this is a good idea.
The extra hours would give you an advantage over the majority of 200 hour pilots.
One thing is, dont go for the cheapest flight training in the USA, you are only cheating yourself as the standards will be lower. Go to someplace like FlightSafety Academy, who have a great reputation throughout the world, for providing high class training.
Also I would aim for doing 12 months of instructing, get a few more hours than those 600. A j1 visa gives you 2 years in the USA, use it to its full!
Have you thought about doing your JAA ATPL exams before you go to the States, this way, when you get back you can convert quickly and get applying to the airlines whilst your flying standards are at their highest, if you know what I mean.
Good luck with what you choose.