There's a few issues with using a model like that.
The first is that if you look at the very last wip screenshot, the model is incredibly complicated, and such a high-poly model isn't reasonable in the source engine. Especially if you want to place 2 or 3 next to one another. All the little details there in the tank's treads would be made of so many triangles you wouldn't be able to do much with it but look at the model by itself.
Even the artist himself states that it's 280,000 polygons--and he didn't say triangles (everything in the source engine is made up of triangles, even world geometry). I'll assume that these are still quads. It's extremely difficult to UV map triangles, so generally I keep them quads until the model is completely finished, then I divide into triangles. So imagine that even at 280,000 quads, those divide into at least double the number when making triangles.
But let's assume 280,000 refers to triangles anyway, since there's no way to tell. The payload bomb cart model has 11,456 polygons, and it's a very complicated model--this tank model has so many polygons, it would be made up of 24.4 payload bomb cart models. I don't know about you, but I think with a full server and a map with moderate optimization, staring at 24.4 bomb cart models would make my FPS pretty sluggish.
And my other issue with using that model is that even though TF2 is a very stylized game, all the objects in it are based in real life...the time period is very specific, and with a tank that's just so 'cartoony', I really don't think it would fit in well at all. What you do still needs to be based in reality in some respects for it to work with a game like Team Fortress 2.
Those things aside, it's still a very nice model, and would be great for 3-D renders, stills and the like. But it's no good in TF2.