Hello Ben,
I have read a few reports which stated MH370 left the gate with 16121 US Gallons the figure I have been using BUT as is the case here the Malaysians don't really know!!!. Although I do use REX, on this occasion I just used the FSX default for weather on 8th March in that region.

My simulation, a stock CS 772 ER (Malaysian RR livery) with a payload of 71900 lbs (approx for 239 POB and cargo allowance), departed 00:41 as per known data, I used 32L.

After IGREX I just set my next WP as the South Pole hence crashing at the same longatude as IGREX = E94deg 25.0' but S35deg 8.0' which is close to the current search zone.

Still playing with all parameters between a few beers.  :P

BTW - I actually flew out of KL - WMKK just prior at 00:05 the same night on an Air Asia flight to Perth WA. OK different terminals but I could have quite easily rubbed shoulders with some of the victims, doesn't bare thinking about!

EDIT: And now, turning my mind back, we departed from 14R on that flight so I would have to assume MH370 left in the same direction.

EDIT AGAIN: Looks like they departed 32R just seen this on AV Herald.
http://avherald.com/files/Audio%20Transcript_MH370%20Pilot-ATC_BIT.pdf