Speed rating on the tires is not the total consideration. Yes you likely won't drive 118 MPH for extended periods.
The 2002 LS 430 I just bought, my first Lexus after many years of MB, has Bridgestone Turanza's that are H rated. H rated tires are the minimum rated tires I would use. An H rated tire gives you better sidewall strength and with the performance capabilities of a LS 400 or 430 that's a factor....Z rated tires are too expensive and nothing more than high priced erasers on your car. These Bridgestones are very noisy....MXV4 are great tires but pricey...check out the new version of Michelin's
XGT Pilot tire..now called something else..They are H rated...priced well...and they perform very well....I have a set on my E300 diesel