Only the original short 4-1 type is cast iron I think. The Wira SE 4-2-1 is not.
The original 4-2-1 is for the early batches of Wira SE. I can't be too sure for those found in Wira 1.6 and 1.8 though. However, I do notice certain carb 1.3 and 1.5 came with short 4-2-1 extractor.
My revs are indeed smoother after I did the piping. My setup won't sound like pranggg pranggg but the sound is definitely a bit louder than stock when I hear it from outside. One more thing is, I noticed the low frequency bass-like humming noise is louder at certain mid range RPM like between 3-4k rpm when I release the throttle. Might be due to the resonator / bullet thing that I replaced my cat con with.