If we closed the notorious multinational corp loopholes, how much could we lower the overall business tax rate?

The issue is not "loopholes". First of all all those "loopholes" were put there for specific reasons by the Congress (not saying they're good reasons, but they didn't get there by chance). The issue is the marginal rate of taxation. If corporate tax rates were, say, 15%, it would not pay to spend billions of dollars to avoid them. They would just pay. In Ireland for example, the tax rate is 15%. Corporate taxes as a percent to GDP are higher in Ireland than they are in the US where marginal rates can be as high as 50% (depending on what state you're in).