Making IF function more efficient

A good solution to repetition is often abstraction.

Dim src As Range
Set src = Me.Range(Dev_Status & "6:" & Dev_Status & "1000")

The Intersect function yields a Range object reference that is Nothing when the specified arguments don't intersect, and a Range representing the intersecting cells when they do. Assuming that code lives in some Worksheet_Change handler (and thus that Me is the Worksheet being handled) and Target is validated to only ever be a single cell, then we should be evaluating the intersection once:

Dim intersecting As Range
Set intersecting = Intersect(Target, Me.Range(Dev_Status & "6:" & Dev_Status & "1000"))

The code should handle that range being Nothing:

If intersecting Is Nothing Then Exit Sub

And then its value is safe to compare... or is it? If the cell contains a worksheet error value, its data type will be Variant/Error, and any kind of operation we do with that data type that doesn't involve Variant/Error operands on both sides of the operator, will throw a type mismatch error. So we should bail in that case too:

If IsError(intersecting.Value) Then Exit Sub

Now we can turn that repeated If...ElseIf...End If block into a Select Case block:

Select Case intersecting.Value

    Case Dev_Raised_IN
        Target_Column = Dev_Raised

    Case Dev_Draft_IN
        Target_Column = Dev_Draft

    Case Dev_Review_IN 
        Target_Column = Dev_Review

    Case Dev_Comments_IN 
        Target_Column = Dev_Comments

    Case Dev_Approved_IN 
        Target_Column = Dev_Approved

    Case Else
        'we don't have a target column:
        Target_Column = -1

End Select

The entire block can then be further streamlined into a simple one-liner key lookup, using either a keyed Collection, or a Dictionary.

Of course, such a collection needs to be initialized, but that can be made to happen once with a Static local, like so (first run enters the conditional, second run doesn't):

Static targetColumns As Collection
If targetColumns Is Nothing Then
    Set targetColumns = New Collection
    targetColumns.Add Dev_Raised, Dev_Raised_IN
    targetColumns.Add Dev_Draft, Dev_Draft_IN
    targetColumns.Add Dev_Review, Dev_Review_IN
    targetColumns.Add Dev_Comments, Dev_Comments_IN
    targetColumns.Add Dev_Approved, Dev_Approved_IN
End If

On Error Resume Next '"key not found"
Target_Column = targetColumns(intersecting.Value)
If Err.Number <> 0 Then Target_Column = -1
On Error GoTo 0