Hey wuzak,
re your question: "Could you make a geodetic wing as thin as the Spitfire's?"
The short engineering answer is yes. The long answer is yes and...
Two significantly pluses would be much greater rigidity (probably solving the wing twist problem of the actual wing) and greater resistance to damage.
A possible third plus would be the ease of fitting additional fuel tanks in the wings, though it is probable that the same problem of where to fit them would apply to a geodetic wing.
A significant minus would be loss of speed (both level and max safe diving) if fabric covering was used.
However, there is no engineering or aerodynamic reason that the geodetic wing could not be incorporated into a stressed skin/monocoque structure. Such a wing could be even stronger, more rigid, and more resistant to damage than a Vickers style geodetic wing. Depending on the trade-offs accepted, it could be comparable in weight to the actual Spitfire wing.
The two questions I have as to the above is what would be the actual weight difference relative to the actual Spitfire wing? - and what would be the cost difference? I do not think that the weight difference would be more than plus 200-300 lbs in the worse case scenario. The cost of production would probably be similar - ie significantly more expensive for the tooling, a bit more expensive in terms of material/geodetic structural parts, but significantly less expensive in terms of man hours (as was found true with the Wellington), or possibly a bit more expensive overall - but repair would be simpler and quicker as long as the geodetic parts were available (as was found true with the Wellington). Parts inventory in the field might be more difficult. (I have never run across any information as to the difficulty of parts supply for the Wellington in the field, but if anyone else has I would appreciate the info, even if it is just anecdotal.)
I believe all of the above could apply to a geodetic Spitfire fuselage also, except that the weight could probably be reduced relative to the actual Spitfire fuselage, though probably only by 200-300 lbs.
Please note that these are just my estimates based on some quick comparisons/calculations and could be wrong (and I am assuming the use similar light alloys to those used in the structure of the the Wellington) though I do not think they would be excessively far off.