The re-designed lamps you are referring to DO have re-positioned filaments. The filament placement is to maximize the 'hot spot' in the focal point of the parabolic reflector, thus giving more light pushing out of the reflector, rather than bouncing around inside.
The trace of xenon gas it not to boost output of the light, but to serve as halogen A/C for the bulb. The bulb stays cooler with the xenon, thus stabilizing the output to a 'cooler' wavelength which produces more visible (useable) light. This will also reduce the UV glare that we see from cheaper lamps.
You are very correct about the PIAA's burning hotter. They have light output all over the spectrum and the heat they generate causes pre-mature failure when used in touring aplications. The best use for PIAA's is in off-road vehicles where light output is supposed to be diffuse and life of a bulb is non-relevant.
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles
[This message has been edited by WmHarlow (edited 10-11-2000).]