1. IQ tests are very good proxies for intelligence. Despite decades of ideology-based objections, the relevant fields have reached consensus that IQ does measure a general factor of intelligence, and it is a good predictor of future scholastic and career ability.
2. My system isn't an alternative to perfection, it's an alternative to democracy which is decidedly imperfect. Intelligence is likely a better proxy for pro-social attitudes than the ability to navigate political institutions and then trick a bunch of people into voting for you.
3. Again, compare this not to perfection but to what we currently have.
4. Training for IQ tests has very limited returns. But as I specified, the test will be used outside of politics, too. In fact its most common use would be by academic institutions and employers. Because of this, most people will have incentives to train to whatever extent possible. Thus the test becomes slightly less G-loaded and slightly more a test of conscientiousness, but any plan to train up a bunch of corrupt lackeys would have very poor expected value.
5. Again, compare not to perfection but to what we actually have now.
