I have recently acquired a S/H Zeatek aluminium undertay for my Alfa 156 1.8TS, 2000 reg. It was actually for a JTD, but the only difference was an extra cutout on the front corner which I covered with some ali sheet/pop rivets etc.
Fortunately all the mounting positions on my car were OK, just needed cleaning/oiling etc. You have to use the correct Alfa Torx bolts, as they are special Alfa bits ( but cheap enough from a main Alfa dealer).
The reason I wanted an ali tray was to keep all the winter salt/grit/water out of the engine bay, especially from the auxilliary belt/timing belt area...( after experiencing a thrown aux belt due to a failed tensioner, resulting in a snapped cam belt, 16 new valves, skimmed head ect etc...!) ; this was probably caused by salt etc being thrown up onto the belt & corroding the tensioner )
I also bought the correct plastic side covers so that when all fitted, the engine bay is protected from the worst of this winter weather.
The Zeatek one is a really good fit, and will always have a value secondhand. Much better than the OE plastic ones which apparently sag with the exhaust heat over time, then get ripped off eventually.
Benefit also is the reduction in wind noise at motorway speeds, but of course the tray has to be removed to do an oil/filter change...
Now that the underside of the car is now 'flush', I was hoping for some slight improvement in economy...but it is still around 31 mpg !
Of course others on this forum have said that they have run Alfa 156s for years without any undertray... so the choice is yours...!