In my corner of the woods plenty of shops offer such services, usually for car hi-fi enthusiasts, but also just to lower the road noise etc.
The noise issue has not been widely discussed at this forum and I think it's because most members are in the USA, where the speed limits are very low by European standards... In my experience, the noise becomes annoying above 75mph (in my country it's legal to drive up to 95mph on highways, and next door - in Germany - there is no limit at all on most highways, you are welcome to try Mazda's top advertised speed of almost 140mph).
Here, in Poland, many GJ owners complain about the noise on a local forum where I'm the moderator of GJ technical section. One of those owners already had the weatherstipping replaced in a specialized car "quieting" shop, but only in one door - the driver's. Not much improvement apparently. He says that the door sounds much more expensive when slammed shut, that's all
The shop I mentioned in the previous post told me that I must fill in the weatherstripping in all doors and, preferably, deaden all wheel wells to really make the difference.
Some shops do not fill the existing weatherstripping with foam, but replace it completely or even replace it with a double seal.
Anyway, four more GJ owners from my forum are going to do it in near future, we'll see what are their experiences. I'll be happy to report it here.
Also, I'm just about to hit the road on a business trip to another city and I intend to measure the noise spectrum inside Mazda 6 at various speeds. Just for fun and out of curiosity. I planned to borrow an AC power inverter and actually take some professional sound recording equipment from my home recording studio, but I run out of time - I'm leaving soon. So I'll just record the noise on a portable recorder and analyze it in Reaper after my return next week. It should give us a clue as to what type of noise it is - more like wind noise (wide spectrum, white-noise like) or with a strong component of the tyres noise (lower spectrum, more peaky).
I will write here what I found