The usual quick-and-dirty workaround when `ItemSize` does not do what is expected is to wrap contents into an invisible frame. One can use `Framed` with `FrameStyle->None` to hide the frame or `Pane`. In both cases, a magic number is required to convert between line-height of `Grid` and printers points of `Framed`/`Pane` - it is highly platform-specific (see [here][1]). The simplified example with `Pane`:

    dot = Graphics[{Pink, Disk[]}, ImageSize -> 40];
    
    magic = 10;
    
    tab = {
       {"Type", "Name", \[SpanFromLeft], \[SpanFromLeft], dot, 
        "Esculap", \[SpanFromLeft], \[SpanFromLeft]},
       {\[SpanFromAbove], "Dia.", 
        "Spec", \[SpanFromLeft], \[SpanFromAbove], 8.3 "mm", 
        Pane[11, {Automatic, 2.7*magic}, Alignment -> Center], "6.3"},
       {dot, "Flycatcher", \[SpanFromLeft], \[SpanFromLeft], dot, 
        "Apus", \[SpanFromLeft], \[SpanFromLeft]},
       {\[SpanFromAbove], 6.9 "mm", 17, "9.8", \[SpanFromAbove], 7.8 "mm",
         Item[8, ItemSize -> {Automatic, 2.7}], "6.7"}
       };
    
    Grid[tab, Spacings -> {Automatic, {{0, -1.1}}}, 
     Alignment -> {Center, Center}, Dividers -> All]

![Mathematica graphics](https://i.sstatic.net/MNo4C.png)

One can get rid of the overhang (gap with missing dividers) by wrapping `Item` around the cell:

    Item[Pane[11, {Automatic, 2.7*magic}, Alignment -> Center]]

![Mathematica graphics](https://i.sstatic.net/VW4aJ.png)

This issue of gaps is only present in version 8. My guess is that as [`ContentPadding -> True`][2] was added to `Pane` in version 8 only, in version 7 a different behaviour is the default for padding inside `Pane` that does not introduce these gaps. However, calling the above example with `ContentPadding -> False` in v8 does not remove the gaps contradily to my expectations.

An almost identical result can be obtained if given cell is wrapped in `Framed` (note the horizontal increase in size of the cell):

    Framed[11, ImageSize -> {Automatic, 2.7*magic}, FrameStyle -> None]

![Mathematica graphics](https://i.sstatic.net/Tb1PA.png)


  [1]: https://mathematica.stackexchange.com/q/1584/89
  [2]: http://reference.wolfram.com/mathematica/ref/ContentPadding.html