Good luck and 354km/h BEAT THAT!
Even though I got nothing against
http://dl.dropbox.com/u/38901452/sa-mp-032.png
Hello sir.
But then again to the original poster:

I actually got to as high as 457 km/h, but forgot to take a pic at the best attempt. If anyone is wondering how is it done-
STEP 1: Get as heavy and fast plane as you can (Don't know if AT-400 is still there, I did this with Andromada).
STEP 2: Get as high as you can with it!
STEP 3: At the maximum altitude type /engine and start free-falling.
Thats it! It's all just about from how high you fall! Also beaware of the fact, that due to lag the spedometer/asl won't update the info as frequently as wanted. In my picture you can see that I had 450km/h on altitude of 123, not 0 as it should have been. Have fun! I am sure 460+ is possible!
Thanks to Lustigkurre who came up with the idea!