One other remote possibility; the jet is a light press fit into the carb body. If it has somehow gotten moved too low, it may be impossible to lean the mixture enough by lowering the needle. I can't imagine how it would happen, but I've heard simiar reports before.
Also on the subject of jet/needle wear; we are literally talking just a few thousanths of an inch, which is difficult to detect with the naked eye. If all else fails, it might be worth just replacing them.
Many years ago, I struggled with a TR3A that always ran hot at freeway speeds. Took a long time (and a blown engine), but I finally figured out that the problem was worn jets in the carb. We had replaced the needles on general principles, but not the jets. On a TR3A there is a lot more range of mixture adjustment; and the jet wear affected mostly idle mixture. By setting the idle mixture "right" with the worn jets, the cruise mixture was way too lean.
Probably should have realized the problem when I discovered the exhaust manifold glowing red after a freeway run; but I was young and foolish then and didn't realize the significance.
My buddy's MGA was even worse, it ran rich at idle even with the jets all the way up. But you could do a "plug cut" and see that it was lean at speed (which is part of what led me to the problem with Dad's TR3A).
The spring-loaded needles are actually supposed to solve the wear problem rather than aggravate it. Earlier carbs (like the SU H6 on the TR3) used hard-mounted needles and centering the jet to the needle was an important step in adjusting the carbs. Not getting it right would result in fairly rapid wear because of the large amount of force applied when the piston fell and wedged the needle against the side of the jet opening. The spring-loaded needles aren't supposed to rub hard enough to wear.
But that doesn't mean it can't happen, after enough decades of operation.