The sky is the limit is how much time and money you wish to spend. They are a couple of easy to apply low cost products that I have used successfully. Will they cut your road "noise" by 1/3? Not even close unless you are willing to spend on the wrong side (IMO) of $500 to do that.When you say "add insulation", what specifically could I look for that won't void my warranty (I'm in Canada btw)? The road noise is definitely annoying, though not by any means the end of the world. If I could lower the road noise for minimal investment, I would definitely consider it.
I just bought a Japanese made Mazda 3 with 18" wheels (UK spec). This car does have hood insulation plus insulation against the bulkhead, a stiff felt like material inside the rear wheel arches and some sound deadening stuck to the trunk floor. Despite this, I find the Dunlop Sport Max TT tyres noisy on rough roads and the Mazda 2I have a base model 3, and it has no hood sound insulation. Do uplevel trims have this? Wondering how much impact this has on in-vehicle noise levels.