The m2eclipse plugin doesn’t use Eclipse defaults, the m2eclipse plugin derives the settings from the POM. So if you want a Maven project to be configured to use the latest version of Java when imported under Eclipse, configure the maven-compiler-plugin appropriately:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>2.1</version>
<configuration>
<source>1.6</source>
<target>1.6</target>
</configuration>
</plugin>