Thanks guys. Quite often as the saw got older but with the saw running at it's best, I did find that on larger logs, it would tend to bog down halfway through the cut on our very hard gums, so an upgrade to the carb and a lower profile chain might be just the thing. With the great power to weight ratio (and low bulk) of the 250, these mods could turn it into the ultimate saw for me. Even it's direct replacement, the newer 251, is heavier and bulkier and at $700 US in Australia, is a lot of money and quite a lot more (nearly double) than they cost in the US.
My other option could be to get a new 250 from the US (they are discontinued in Australia but still made in the US) for around $320 US. 'Cos the risk is as someone mentioned, I could replace the carb only to find I have engine issues, which is something I have never re-built before on anything. And at the end of the day I will still have a 10 year old saw....and my guess is that they weren't as well built then, as the older saws people are keeping alive.
My only concern with a new 250 is that they will have messed around with the carb and muffler for EPA regs, making it less of a saw than the one I have?
Plus I have the very real problem of finding a Stihl dealer who will a) post me a new saw (they usually insist on the buyer being there and getting a demo on the saw) and which has never had fuel in it (for air freight regs).
Perhaps the most attractive option is to get a new 250 from the US and change out the carb to the Walbro, mod the muffler and change out the B&C. The only remaining issue I would have to solve would be the freight problem.