Move subsite to another Site Collection

Hi,

I'm trying to move a subsite from one site collection to another using the PowerShell cmdlets export-spweb and import-spweb (with the includeusersecurity switch).

This works to an extent, but is missing several important customisations (security groups, custom forms, workflows, site theme). 

Is this the best way to move a subsite to an existing Site Collection?

Many Thanks,

Iain