True enough, the standard manifold/downpipe setup flows unusually well for a stock system. I recall Mike Barrett, at British Leyland back in 1969 or 70, saying so during a seminar he ran on emissions which was held for dealer technicians at USA BL headquarters in Leonia, New Jersey. Well! that shows how stinking old I am, but at least I still remember! There are headers out there, Peco among them, that do show better flow on a dynomometer, though not all THAT much better. If you were going to use a B motor in competition you would certainly want to use one. The difference is much more "real" when the cylinder head has been ported and reworked with bigger valves, etc.
It's easy to assume that an aftermarket header would be a performance improvement just by looking at one and thinking in racecar terms, but the INSIDE and the runner combinations/lengths of the system, regardless of the material from which it's constructed, is what counts. One advantage of the stock manifold is that it will resist cracking and general deterioration much better. Of course, this is all true concerning the PRE 1975 single carburetter combination intake/exhaust manifold, which flows about as well as maple syrup at minus 200 degrees centigrade.
If it is your intention to use a steel tube header, I heartily reccomend having it ceramic coated inside and out by one of the companies that offer this service. This will not only make the unit MUCH more resistant to heat cracking, but it will also lower the under-bonnet temperature considerably; say 20 or 25 degrees. That counts a lot with the SUs living just upstairs from the exhaust.
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon