Fastest method of getting the bounding box containing all value ranges
This should work. It's pretty simple method. I cannot test with your sheet as mine is not nearly that large, but you can try it.

IMPORTANT NOTE: This MUST be called as a Sub if it is a function that is called from a cell then, it will not work. (because excel uses the active range to perform the start of the SpecialCells I believe).

I have tested this code with your picture of cells and it works well.

It also works with other versions of cell combinations and locations that I have tested.

The reason to use the following code is because it is simple and can save valuable programming time.  The SpecialCells Method is reliable if used correctly just like any other programming language and function. It is worth a try and also worth timing with your larger data. 

In addition, the OP also uses SpecialCells in his code, just not the same way that I have.

I hope this helps.

    Option Explicit

    Sub RunIT()
        'Input range must be the first cell of where the data starts
        GetAllCells Range("A1")
    End Sub

    Public Sub GetAllCells(rngInStartingRange As Range)
        Dim strTemp As String

        strTemp = Range(rngInStartingRange, rngInStartingRange.SpecialCells(xlLastCell, xlTextValues)).Address
    End Sub