GOP structure is the same as the other Sony HDV's.

However the chance of anyone editing in the native format is unlikely. Remember that Discovery has said that XDCAM HD is accepted for unrestricted acquisition. In editing most will transcode to a different format. Sticking to the MPEG format for editing is silly, especially for broadcast, not least because of all the uneven GOPs that are created.

Until I work out my exact requirements I am using a Canon 19x6.7 half inch SD lens. It's the VCL719BX, the lens supplied with the HDC-X310 HD camera.
Shooting a test chart I am getting better than 700lph, even in the corners and CA is not objectionable. The half inch lens is certainly sharper than a Fujinon 17x7 SD broadcast 2/3 lens with adapter.

I'm still undecided as whether to get a 1/2 inch or 2/3 inch HD lens, if at all. I may just rent in as required.

Why are there such restrictions for HD when home-made looking stuff like American Chopper, Monster Garage, Dirty Jobs etc... make it to the air?

Because, it's not how good something is--it's how well connected the producer is.

Beyond that, those are reall gritty reality shows and HD isn't as important.