Plenty of people have fitted 20" rims (open the link I posted and you'll see plenty on 250/350's), although IMO that's extreme as I have to be careful with my 19" because of the lower profile tire. What do you mean 'speed difference'? There should not be a difference (within the tolerances of what's available) as the bigger rims sizes use a lower profile tire to keep the rolling diameter i.e. the outside diameter of the tyre the same as before...applies whether you have 16", 17", 18", 19" or 20" on the IS250/350. There's only a problem if you fit a much wider tire or use rims with unsuitable offset.
My rolling diameter is slightly smaller than with the 17" rims and tires I had on the car originally, and makes a difference of around -4 mph at 60 mph from the OEM setup, but as this is within the usual +10% the speedometers read anyway it just makes it more accurate. I've verified this with a hand held Satnav over a 5 mile straight line so haven't bothered to recalibrate the Lexus Satnav I have in the car (which apparently is possible). I switch between the 17" rims and 19" for Winter/Summer driving anyway.
The only other difference is a slightly harder ride due to the lower profile tire not having the same amount of shock absorbance as the original tires. There's no other change in the way the car feels as far as stability is concerned.
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I missunderstood you.
But if use larger tire size the speed would be higher even your speedo shows lower speed and I'm interested in knowing the difference.
There are various calculators to help you match up profiles to get similar speedo readings.
Here's one:
http://www.net-comber.com/tirecalc.html
http://www.moparregister.com/articles/calc/