| The fluting in the glass doesn't affect the light output if you've correctly mounted the projector and its cut off in the correct location within the assembly. Depending on the retrofit projector this might not be possible, thus you may have a nice projector with wonderful light output but with the glass lens on it doesn't conform to the formula the engineers who designed the headlights as the projector can't be mounted correctly to account for the fluting. The fluting in the glass is there to reduce glare onto oncoming traffic while maximizing the best light output as possible while being safe. Many of today's headlight don't have many of these flutes as they are able to reduce the glare with other means inside the projector and/or with auto leveling etc. The OEM Xenon lights use the same glass as the halogen jdm and euro spec. Late model USDM headlights actually has additional fluting in the glass...directly in front of the projectors (and any new light installed on a Z after 95 even as a replacement will have this revised glass with additional fluting). You have to keep in mind every part of a headlight is designed around light output that is not just maximized for better visibility but also safe for oncoming drivers. Once you start mixing parts you change this drastically. Unless you plan to spend a lot of time trying different projectors and testing them in different mounting locations it's going to be tough to get as good of a result as you'll get with the out of the box with OEM Xenon. The light output is already very good as is the cutoff clean with the OEM Xenon. Your assumption that the glass lens likely affects the light output on the OEM Xenons is not correct at all. With these the only thing left to do is correct the cutoff for LHD. Here is a photo the Xenons on my Z after making the cut off correction. You'll see the cutoff is good.
 This photo shows the light output on a dark road, photo taken from inside the car.
 Getting lenses made without the fluting will allow you to almost use any projector that you can get to fit within the assembly and you'll likely have that clean cut off you're after but you'll also likely send a lot of glare onto oncoming traffic. A note about the OEM Xenon bezels, you mentioned. Whether you source OEM pieces or buy aftermarket replicas...you'll have a few issues to keep in mind when using them. 1. The OEM Xenon projector is larger than the halogen, thus the circle opening in the bezel is larger. If the bezel is placed inside the halogen assembly with the halogen projector you'll have large gaps around the projector that in my opinion look horrible. Depending on the size of the projector you end up using this is something to keep in mind. If you use a projector that is similar in size to the OEM halogen projector you'll like want to build an additional custom bezel to fill in the gap. 2. The high beam reflector in the OEM Xenon assemblies is smaller than the Z32 halogen high reflector. When placing the Xenon bezel or any replica of it in the halogen lamp assembly you'll cut off the lower portion of the high beam reflector, as the bezel is raised in this area in front of the high reflector. While it probably has some negative affect on the high beam light output it just doesn't look correct to have it covered up.
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Some thoughts - 
