It has been told around ice racing campfires that Bob Legere once upon a time mentioned that lengthening the headpipe by 10 inches would be beneficial. However, if that was ever actually put into writing, it and many other Opel legends were locked away in a vault. Your quest, if you choose to undertake it, will be to obtain the secrets of the Opel Masters and publish them in a forum so the entire world will know.
I have made mention of lengthening the pipes of an aftermarket 4-into-1 tubular header by 10", but not a stock headpipe.
However, in Europe the stock headpipes are commonly modified for various use. For a semi-competition engine, the actually shorten the twin downpipes to move the torque a bit higher into the powerband, as well as increase peak hp a bit. Conversely, they DO lengthen the pipes at times, but how much....and to what effect I do NOT know. I have never tested this theory with the OEM style downpipes, only with headers.
If you have the talent and time, I say do back-to-back testing and report it to us on this list. Don't bother with 0-60 testing, it's more traction-derived than anything. Choose a gear, say 3rd gear, and test acceleration from 50-70 mph. This is a good torque/flexibility test. Make 10 runs or so, and average the times out. Keep note of the best time too. Then, modify the pipe, and do the same test again with the new combination. Try to keep it fair and shoot for two days with similar temps and humidity levels.
This would be a fair assessment, and would justify (or not) the time to do the modifications.