As far as I have experienced: VERY TRUE!!
I have a turbocharged 1303 (standard fenders) and have recently done a run with the revlimiter on 6500 RPM (was 5700 RPM) which enabled a (theoratical) top speed of 240 km/h.
I originally upped the revlimiter to check if the 'impossible' setup with single springs and custom camshaft would be able to turn 6500 rpm without valve float.
Since I had no problems with valve float, I decided to see if it would still pull above 200 km/h and floored it in 5th gear. I was surprised how easily the engine revved up to the redline (the gearshift light came on) and I lifted.....
Then the **** hit the fan: I had the feeling of driving on ice, and had little to no control over the car (needed both lanes to keep it on the road).....
Luckily the car stabilized at about 200 km/h and I carefully applied the brakes.
To be honest, I was scared ****less and I thank God that I survived this incedent.
IMO stock fenders create quite alot of lift and I think that caused the scary ride......
I don't know if louvering the fenders would help, but if you're planing on 200+ km/h (or less in a lightned bug), I'd sure as hell try it!
Grt. Robin