I have 1150GSA and Gl1800.
I fitted an aerofoil to top of 1150GSA screen and find this made a big difference. I use sys 4 helmet and moulded Ultimate plugs.
Believe it or not, the wind noise and buffeting on the GL1800 is much worse than the BM, such that I spent all yesterday testing the 3 different flip fronts that I have whilst riding the wing, including the brand new Airoh SV55 which I bought on Saturday.
The SV55 is the noisiest of the 3, but the vents are the best and the integral sun visor ( first I have owned ) is excellent. Surprising quality given the £130 price tag.
The Sys 4 is 3 years old and as such the lining has compressed, making the helmet noisier than new. There is little noise at low speed, but at 50 and above the helmet buffeting and deep droaning noise is the worst of the 3.
The quietest is the Shoei Syncrotech, which I dusted off for the test. A number of years old and now the styling is not as good as the newer models, but certainly the quietest and possibly the best overall fit.
All that said, the BW is a different bike and next weekend I will repeat the test on the GSA. I'll report back.
Incidentally, tried the Sys5 at Williams a few weeks ago, only in the shop, so not test driven it, but the quietness which BM say they have achieved is partly a factor of the size of shell they have used, which on my large head, looked like a chemical toilet with the front up ! I may ask to test drive, but in the meantime, I would value your opinion of the Sys 5.
Regards