I second this thread -
https://www.vwvortex.com/threads/borla-exhaust-metallic-whistle-going-insane.9467864/
I'm heavily involved in this topic as well as the thread above. Basically thread jacked it, but for good reason. I've been in correspondence with Chris Baas at Borla discussing this issue thoroughly as I was one of the first people with this exhaust on a MK7.5 R. Every single system I've heard for a 7.5 has had a whistle at WOT. Some are worse than others, but they all have the sound.
This is Borla's press video - The sound quality is terrible and they need a new media director, but that's a different topic. Listen closely as this car builds speed and you can hear the whistle. It's not overly pronounced in this video, but can clearly be heard.
https://www.borla.com/products/volkswagen-golf-r-cat-back-exhaust-system-140830sb
Chris went as far as sending me out a replacement system which I installed only to find that it was even worse than the first. Whistle was loud and could be heard at varying RPMs under hard throttle. It's a shame as Borla makes one of the best sounding systems for a cat back w/ stock DP on the market. There is zero drone... I mean zero.
I've been in contact with several different people about this topic on varying platforms including YouTube, different forums, and via email. Some people are telling me that Borla's engineering department has found what's causing the issue from a technical perspective, but I'll believe it when I hear it. They told me the same thing when shipping my replacement system out only to find the opposite. I've not heard from Borla on this after shipping the replacement system back over a month ago even though others have received reports. I emailed Chris on Sunday and haven't received a reply yet. Talk about a nightmare. The amount of time I've wasted going back and forth with Borla, removing and reinstalling systems from the car, etc. I'd say you are better off going with a different company.
After the ridiculous amount of research I've done on exhaust systems available for the MK7 / 7.5 R I'd be willing to offer some advise on what I think the best system is for your situation, but you specifically stated you didn't want that.