Literally every government for the last 35 years has promised to get tax compliance up. It's not gonna happen. Beware Greeks Bearing Bonds[0] goes into good detail on the problems with the enforcement bureaucracy and legal systems that make it so difficult to actually apply the law.

[0] http://www.vanityfair.com/business/features/2010/10/greeks-b...

Higher tax compliance would help the government pay their bills, but it would in equal measure harm the people's ability to pay their bills.

Greece's biggest problems are not high debt payments, they are 25% unemployment, and GDP having dropped 27% from its peak.