The theory behind the framerate lock in Flight Simulator is to free up processor to do "under the hood" tasks during low demand graphics requirements.
It was described in a FAQ I have as follows:
It is said that: for example, you lock it at 25 fps, then the framerate will not exceed 25 frames per second. Why would anyone do that? Well, when you have locked the framerate, the computer will have more processing power for other processes, like the weather, AI traffic, etc. This will make FSX run more efficient, so you the overall flight sim expirience will be better. Ofcourse, this is only relevant if your pc is capable of generating more frames per second than you've set the limit to.
It is my personal belief, based on what I have read in the former developer blogs and elsewhere, that this "lock" may not be helpful on multicore processors with high speed GPUs.
I would suggest trying with the lock, and with the setting on unlimited and see which works best . . . often I find the unlimited works better for me.
Intel i7 950 @ 3.9 GHz
MSI 560 Ti Twin Frozr x2 SLI @ 950 mhz
6 GB of Corsair memory (timings unknown atm)
H50 CPU cooler
3dMark11 P7800 at stock speed, P8643