I have indeed had both on my car - having kept the Invidia.
Quicksilver was a good "louder than OEM" sound and is better than OEM for sure. It did drone a low revs up hills and certain speeds on the motorway. I can't describe the feeling very well by typing on here, but I felt I had compromised the Lexus quality by adding this exhaust. That's no reflection of the product itself which is very well made, just a feeling I had as an owner driving it around.
I wasn't a fan of the carbon tips - some would prefer them which is fine, they do look the part if you are looking for a more aggressive overall look, but they didnt suit the back of my car in blue.
So as I wanted to keep the OEM look on the rear I went for Invidia as it is also Cat Back for the price of the Quicksilver axle back.
Never regretted my decision. The Invidia is just as aftermarket as the Quicksilver of course it is, BUT, it does not feel like it and honestly came away from the garage feeling like it had always been fitted from factory. It's quiet in the low revs but with added V8 burble which the OEM misses and it kicks ***** and turns heads when you put your foot down! I have had many compare the sound to an Aston and I tend to agree.
I have experience zero drone, zero.
If you give me a couple of mins I will upload a drive away vid which a friend of mine took of the Invidia.