Jan,
Of course there is a best length. I have run a series of tests lately, that confuses you ad infinitum.
For the MVVS header I can tell you that 300mm is about the best for sub6000 rpm. our header seems a lot longer than that.
Thinner headers reduce power, but broaden the power peak
thicker headers shift power band up and narrow the peak
run a series of test using different props. This will show you the peak power rpm (use my prop power calculator, download from my web). Plot the curve, and you will know what to do!