Hello,
Wider fenders would certainly hurt top speed. Take a look at the Ruf CTR, this was at one time the fastest road going car. This was also due to the narrow body Ruf used. If he had used the Turbobody is would have cost him certainly 15 km/h in top speed. But, do you really want to go 240 km/h in a bug?
My racecar lifted almost 10 cm ( about 4") at 210 km/h at the main straight. This was without any spoiler though, and 2" wider fenders. So.....
Cheers,
Richard