I've been using my M900a pretty hard for several years and I haven't ever run into any issues where one ARMS locking lever wasn't strong/strudy enough. IMHO, an extra lever and larger mount would just add unnecessary weight. Seriously, how many broken M900a's have you heard about?
Believe me, I've banged that grip handle pretty hard
a few times and it always held on tightly.
While I believe the LaRue lever is superior one of the reasons I haven't changed over yet is because of the experiences I mentioned above. However, I do feel the little flip-floppy rail buffer on the ARMS mount can be a PIA when mounting the light.
Here's the LaRue LT170 that I mentined earlier.
Info from LaRue's site : Upgrade your SureFire M900, M951 and M961 Weapon Lights with our Adjustable SpeedLever "17 Upgrade" Mount