Out-File seems to force the BOM when using UTF-8:
Out-File
$MyFile = Get-Content $MyPath $MyFile | Out-File -Encoding \"UTF8\" $MyPath
I figured this wouldn't be UTF, but I just found a pretty simple solution that seems to work...
Get-Content path/to/file.ext | out-file -encoding ASCII targetFile.ext
For me this results in a utf-8 without bom file regardless of the source format.