Sunspider benchmarks from other browsers:
N900 @600 Firefox: 16488.6ms
iPhone 3GS Safari: 17360 ms*
Nexus One 14409 ms*
iPad: 10475 ms*
* Source: anandtech.com

It seems that the Firefox javascript engine (Tracemonkey) is a lot better than the one in MicroB, so how about running the test on Firefox @900MHz?