I have just received a bulk supply of 13 watt, 4 pin, Biax D/E lamps made by GE in Hungary, and note that the design has been altered.
These lamps consist of two "U" shaped tubes mounted on a plastic base.
The new ones use noticably thinner tubing and look a little odd compared to the previous design.
I would expect that the newer thinner lamps would give a less even light distribution in fittings with a reflector, though this does not seem noticable in practice.
In cold conditions I would expect initial output to be reduced.
The new lamps are lighter in weight and presumably cheaper to manufacture in view of the reduced materials needed.
The reduced surface area of the glass tubing would reduce the amount of phospor needed, which is an increasingly expensive component for lamp manufacture.
Supplies of rare earth phospors were seriously affected by the Japenese earthqauke and tidal wave disaster.