Raising the tire pressure at higher speeds may be a good idea, but I would advise against it. Case in point: when I bought my 1990 300-D it had 4 Yokahama steel radials on it. I drove from Miami to Atlanta at around 75-80 mph. Somewhere on the way home, I checked the air and filled each tire to 34 psi, according to the gauge at a Texaco station and checked the last tire with my own gauge to make sure the other gauge was accurate. Within 400 miles, each tire got a bulge on it and some serious tread separation. I called Yokohama and they sent me to NTB, where they told Yokohama that I has 42 psi. Yokohama said that they woldn't ,make any adjustment. Then I told them how I was planning a website about how Yokohama didn't stand by their tires and they agreed to compensate me for 75% of the cost of any brand of new tires. I got 4 Dunlops and have had no further problems.
But if you DO overinflate, reduce the pressure and watch them at the tire shop when they check it. I don't think this tread separation is all that rare: I have lost maybe a dozen tires to tread separation in the last 20 years.