As a territory outside the U.S., Guam has an ability to choose and design its own tax system. However, the jurisdiction's current tax legislation is subject to the Internal Revenue Service code. The U.S. tax code which Guam uses is marred by complexities, convoluted and heavily complicated. There's a growing probability that Guam will de-link from U.S. tax code. In fact, the latter maintains some regressive provisions which are inapplicable to Guam.
In case of the separation from U.S. tax code Guam would get an opportunity to adopt flat tax. The tax base would increase, there would be less avoidance, double taxation of income would disappear. The simplification of the tax law would also reduce the costs of tax bureaucracy and administrative costs of tax collection. And most importantly, lower taxes would stimulate the growth of output nevertheless.
Source: De-linking from the U.S. tax code makes sense for Guam, Pacific Daily News, November 18, 2007 (link)