Well, see, that's what I've been saying. Try it first -- you may find that you're plenty satisfied with the onboard recording, and would therefore find other places where that money can be spent that would have a more significant impact on your end results.
The Nano is really quite excellent, but I do have to make a clarification on what you said -- you said "multiple record codecs" and that's not quite accurate, the only codec it offers is MPEG-2. It does offer multiple file formats (.MXF or .MOV), and it offers multiple varieties of MPEG-2 (you can choose the bitrate and whether it's long-GoP or intraframe) but it's all still MPEG-2.
Just to confirm Barry's info on the BBC, a colleague did a shoot for the BBC a couple of weeks ago. Their initial request was for a Sony 650 or the like but nothing less than 4.2.2, however the BBC like many public broadcasters has faced significant budget cuts and what they were willing to pay for the shoot was way below what would be a considerable rate for the camera and op for the day. They insisted on 4.2.2 and finally agreed to an EX3 and NanoFlash as a compromise. Perhaps their Engineering department needs to have a chat with their Programming department.
where you have little control of the scene, lots of movement, panning etc then you'll break down the Avchd codec pretty easily. This is where a better codec comes in, quite logical really that a higher the bit rate of the same or similar codec the more bits of info it can handle and the less blockyness and mush you'll get. I always wondered how some TV shows look so good, everything looks smooth and no horrible blockiness etc. I'm pretty sure a lot of it has to do with the codec they use (sure they have racks of high end hardware H.264 encoders that cost thousands of dollars too)