Limiting the top speed to protect the MGU seems plausible, but Lexus doesn't appear to do this with "bigger" cars like the GS450h or LC500h. They'll do 155mph if required.
On a related subject, I caught the tail end of an old Top Gear episode the other day, where Clarkson was driving the then-new Nissan GT-R, in Japan.
That car was also limited to 180km/h like everything else, but interestingly, the limit was removed when the car knew (via it's navigation system) that it was on the Fuji Speedway racetrack.