I know this is an old post, but I have an answer for it so I thought I would share. I have been able to successfully create a non-default 'NewForm2' file for a list using PowerShell and still retain the original OOTB ability of the people picker to autocomplete.
Like many others I found that creating a new list form using SPD stripped the OOTB people picker for an older version which did not perform as expected. How I solved this problem was to simply not use SharePoint Designer. I used PowerShell to create a new form for the specified list and was pleased to find that the OOTB autopicker functionality for the people picker once again worked as expected. The dropdown populated with the pool of results and I was also able to successfully save the form with the people picker value.
$url = "http://SiteURL/"
$listname = "NameOfList"
$web = get-spweb $url
$list = $web.lists[$listname]
$files = $list.rootfolder.files
$newformurl = $list.RootFolder.ServerRelativeUrl + "/NewForm2.aspx"
$newform = $files.add($newformurl, [Microsoft.SharePoint.SPTemplateFileType]::FormPage)
$wpm = $newform.GetLimitedWebPartManager([System.Web.UI.WebControls.WebParts.PersonalizationScope]::Shared)
$lfw = new-object ([Microsoft.SharePoint.WebPartPages.ListFormwebpart])
$ilist = [Microsoft.SharePoint.WebPartPages.IListWebPart]($lfw)
$ilist.ListId = $list.id
$ilist.PageType = [Microsoft.SharePoint.PAGETYPE]::PAGE_NEWFORM;
$wpm.AddWebPart($lfw, "Main", 1)
$list.DefaultNewFormUrl = $newformurl