I put falken fk510s on my 2nd gen is350, and it was a huge difference in ride quality/quiteness as well..... it is a bang 4 your buck tire and I believe they are made in japan.... check tire rack.com for ratings if you havent already and go from there and last I checked the fk510 was 3rd best and closely behinds general rs summer tires which this fk510 competes against.
Also stock tire size on the 350, to this day when going 50 or under the road noise is barely audible but it depends on which roads you drive on too, if its a fairly new or smooth road I basically hear almost nothing and its crazy.

 

I second these, just put them on 2 weeks ago and have been really happy with all aspects of them except the steering. The responsiveness isn’t as good as my Michelin Pilot Sport A/S 3+‘s or other upper high end tires, but that’s my only gripe.

 

What about when you actually drive the car?
Most of my highway driving is 80+.

 

I drove in some heavy rain yesterday on my Goodyear Eagle Exhilarates and man these are good in rain. I still wonder how they would be on an IS. They are grippy without being loud or harsh. Only downside is they are a bit numb through the steering wheel. I do recommend them!

 

hahaha well I actually dont speed much at all. I mostly go 65 on the highways all the time maybe 70 too and these tires still run quiet. The noise is more noticeable once 80+ but still depends on what type of road it is... when I do go 80 which is rare the tire noise is much more noticeable at that speed but on my highways here in norcal the tarmac types change so its more quiet on some normal highways while the interstate with the white road idk what its made from but those are much more noticeable to road noise.