I also Have Lexar 16gb 400x card, and it is limited with my camera to 45MB/s... !?!

this puzzles me, for you should have no problems recording Raw.
I wounder, what are your setting ?
Maybe there is something no set right of over looked .
these are my setting : Canon Menu
photo=small jpeg
Auto power off=OFF
LCD Brightness=manual
Movie rec. size=1920x1080 24p
ML Raw record module from june29 from A_D_ Hack built (the one with the disclamer)
ETTR=Off
FPS override = set to exact 24
in the Raw Video tab
i set previews to Auto or Hacked
recording frame=White rect
Small hacks= On
I have notice that on my Lexar 1000x 64 GB card i had to Brake in the Card like a pair of shoes

I did about 12 full card records before it started to write at the proper speed ,
and the other thing that you may try is to set warmup card
or just write 200 frame at lower resolution then Start the Main record .
I hope that helps
edit:What is the Max Frame size you can record?
Don't be to concerned about the benchmark read write test,
It's the Frame size you need be concerned about.