Convert a PC path to Mac path with AppleScript
From what I can see, there's nowhere that P: gets replaced by your Mac path smb://perkinswill.net/projects. Adding another replace for that seems to get the script working for me, see below. If you have other servers with other paths, you can add those in in a similar fashion.

Note that by changing the actual opening command from tell application "Finder" to do shell script it also handles spaces in paths, at least in my environment (which is slightly different from yours).

on searchReplace(theText, SearchString, ReplaceString)
    set OldDelims to AppleScript's text item delimiters
    set AppleScript's text item delimiters to SearchString
    set newText to text items of theText
    set AppleScript's text item delimiters to ReplaceString
    set newText to newText as text
    set AppleScript's text item delimiters to OldDelims
    return newText
end searchReplace

on run {input, parameters}
    set myClip to the input
    set mytext to searchReplace(myClip, "&lt;", "")
    set mytext to searchReplace(mytext, "&gt;.", "")
    set mytext to searchReplace(mytext, "&gt;", "")
    set findIt to "\\"
    set replaceIt to "/"
    set mylocation to searchReplace(mytext, findIt, replaceIt)

    -- Add these three lines for any path you wish to be able to open
    set winPath to "P:"
    set macPath to "smb://perkinswill.net/projects"
    set mylocation to searchReplace(mylocation, winPath, macPath)

    do shell script "open " &amp; quoted form of mylocation

    return mylocation
end run