I have either had or have about every addon known to man. I just recovered from a HD failure and after a week have most of FSX back to whre it was. But this time I made sure to test each one to see its effect on performance / stability. A little OT but I will get around to your specific question.
Without a doubt the worst resource hog, fps killer,stuter inducing program was MytrafficX, the latest ver. I now run with no ai traffic and do not miss the long long times and brain dead ai.
I also have loaded Activesky 2012,latest ver and Ultimate terrain. I just did not like the clouds that came with Activesky but their dyanamic weather,runways textures and others are great. So I reloaded Flight Enviro X which imo have the most realistic,easy on the eyes clouds/skies/sunsets. I have always ran Activesky and simply could not do without their dynamic weather. Install can be somewhat of a pita however... There are plenty of options for rez but I stay 512 or 1024 iirc. Anything above just looks to sharp imo.
Topped off with Ground enviro and even on my aging I7,2.8 with 6 gigs, Vista 64 wide screen 37 inch led, Gtx 660ti runs smooth even with the high end addon planes and high FSX graphics settings.
I also run the Bjote prog and then let it be. My days of endless CFG tweaking,moving sliders and chasing the Holy grail optimal settings are long over.
All that said the best thing to increase performance and stabilty was actually when windows got nuked along with FSX forcing me to reload FSX. Reloading I did some much needed house cleaning. I did not realize how much crap had accululated both on my system and FSX. All those progs installed,partially removed,old sceneries,Dlls from who knows where......