Is it a good idea to chop this seat tube?

Personally I think this is a bad idea.

The high seat tube seems designed to compensate for the low rear stays, creating a kind of cantilever effect when the saddle/seatpost is putting load on the seat tube and the lugs/joins.

Shortening the seat tube to where it meets the top tube will negate this and put more stress on the top tube lug area, and possibly the stay welds as well.

I don't know if the difference in force would be enough to cause a problem and it could be compensated for by something as simple as a really long seatpost that descends lower than the stays to ensure the force is spread over a greater area, but it's something that personally I'd not want to do to a brand new bike when if I just shopped around i'd find something that fits straight off the bat.

Mate, this is steel frame so over sized by definition. The basic frame builder at the corner of the street love steel because they don't need to do any stress simulation, add the tube together and you are done. If you think in term of pure force apply to the frame and its direction from the saddle, it is a vertical force equal to the weight of the rider (i simplify). Then the weak point is probably at the connection with the top tube. But the force apply at that connection will depend of the leverage which depend of the distance between the saddle and the top tube plus its angle. If you reduce the distance....less leverage so less force and no extra force applied somewhere else.

So i can't see how it can be a problem, a good learning video to understand the weak points and how to stress a frame:

http://www.youtube.com/watch?v=MWtLIzgxqBQ

good thinking markxrat, you need to do a cut to be able to tighten the clamp. A 2 mins job to cut the tube 1.5cm and it should do the trick. You just need to make sure you leave at least 2cm tube to do that so you avoid cutting too low on the frame.