Yes the IS already has relays.
And, after using 55/60s for years, I installed pairs of off-road 80/100s and 90/100s H4s in my two interstates a few years back, and they have been a great improvement, and I have no heat or wiring or socket issues whatsoever. The IS fairings really breathe well (unlike a std/tourer bucket). I run dims all the time, rarely run at night, and have never been flashed on dims day or night.
This should not be done in a std/tourer bucket without upgrading wiring, socket and relay. MarkT has the information on such an upgrade (and has posted it here before).
Like I said before - plenty of cooling in the std/tourer bucket. In fact I clean bees etc out of the wiring in there when I open it - they come in through the large gap around the ring while the air flows on out the big hole in the back. I run a 130/90 H4 bulb in mine and have since '98. No heat damage at all. My wires are upgraded with relays and the socket is high-power too.
http://www.horseapple.com/Valkyrie/Tech_Tips/Headlight_Upgrade/headlight_upgrade.html