Can we make an HD4850 from a HD4830 card?...

It is clear to see from simple photographs that the HD4830 and HD4850 cards
use the same circuit boards, layout, cooling fan etc. The 4850 has 800
stream processors and the 4830 has 640. I ready recently that some 4830s
were released with a pre-release bios and shipped using just 560 stream
processors. A BIOS update fixed this problem. Given that the BIOS limited
the cards to 560 stream processors, then does the BIOS also limit them to
640. Do the 4830 cards actually have 800 stream processors and are they
therefore BIOS limited to 640 - in other words, could a BIOS be devised that
lifts the limit and gives access to all 800?

Would it be possible to simply flash a 4850 BIOS onto a 4830 card and give
the card access to the full 800 stream processors?

I think the memory speeds on the 2 cards match, so the core speed and number
of stream processors are the only difference. So given that the heatsink and
fan setup looks the same on both cards, then what prevents a core speed
increase to 4850 levels?

So if we can lift the stream processor limit and gain access to all 800 and
also raise the core freq to 4850 speeds, then we get a 4850 for the price of
a 4830, saving £20.

Possible problems:
Are the 4830 cards just 4850 cards that failed stream processor tests, so
are they limited due to faulty processor parts, or is it simply ATI covering
all market price points? Are the 4830 cards from the same production line as
4850 cards and have their stream processors limited physically?