I think more information is needed regarding the school's testing/selection process. If they will take the outside testing, then I would say yes, do it. IF you think he will cooperate, etc. The bonus is that you control those scores and can decide to show them to the school or not. The downside is that they may not accept them, so you definitely need to find that out.
If they won't accept them directly, then you'd want to know what their procedure for identifying gifted kids is. This is where it gets tricky because if they do it based on tests that are different from the private tester's, then even if he tests gifted privately, you don't know for sure that he will meet the school's criteria. If they depend heavily on the CogAT, for example, and your ds is one of the many gifted kids that doesn't score high on that test, then the private testing wouldn't really help with regard to getting in to the magnet school (assuming they won't accept those outside scores).
So, I think to give a good opinion on this, I would need more information about the school's procedures.
If they will accept the outside testing, though, for admission to the magnet school, then I would probably say go ahead and do it if you can afford it. In the long run, the money would be worth it to reduce the uncertainty.
