I have a BAM file that has 33X whole genome coverage.

I would like to create a new BAM file from the original that has 10X genome coverage.

Is this the correct method?

    samtools view -h -s 0.3 in.bam >10x.bam

Also what methods do you use for fast calculation of whole genome coverage? GATK is bloody slow.
Yes, the `-s 0.3` will result in an average 10x coverage. One simple method to get total coverage is to simply parse the output of `samtools depth`.