I think you're right, max windspeed is max windspeed, regardless of storm movement. From two relevent paragraphs at the hurricane hunters webpage:
"Special weather sensors mounted on the WC-130 are tied directly into the IWRS computers, and measure temperature, dewpoint (humidity), barometric pressure, actual
(radar-measured) altitude of the aircraft, and winds to calculate a complete weather observation every 30 seconds. The computers are also connected to the aircraft navigation
system, which adds position to each observation. "
"The sonde measures winds by a rather elegant method. The sonde triangulates its position relative to several different Global Positioning System (GPS) satellites in orbit. If there
were no wind, the sonde would fall straight down, and its position over the earth would not change. However, as the sonde drifts, it calculates the wind speed and wind direction
needed to push the sonde by that amount. "
http://www.hurricanehunters.com/faciwrs.htm
Both statements indicate that the Vortex data, the basis for most subsequent projections, represents data measured relative to a fixed position on earth. Therefore, max wind is max wind, regardless of storm motion.
I clicked Gulf Coast Region, Category 2, Forward Motion (20 knots). It says MAX WIND=090 KT, SPEED OF MOTION=20 KT, and shows a result of 110 KT.