If you did what we did and snapped up a few base model '13 Mac Pro's with 12GB of RAM because they were available when other models weren't, you've probably discovered what we did - they max out in FCP X and Motion 5 pretty easily.
In my other life, I use Transcend memory for a family of server appliances and while discussing new options with our rep, he mentioned that they have a very cost effective memory kit for the new Mac Pro. I thought - hmmm, bumping the 12GB units up to 32GB for $300 would be a smart move, so we had 2 sets of their 32GB kits sent over.
We've been putting two of the 12GB base systems through their paces with the new Transcend kits and all I can say is wow. The background rendering is over 30% faster than the same raw clips with 12GB.
If you've got a 12GB unit and flinch at Apple's upgrade price for 32GB, check out the Transcend "Ultimate Upgrade Kit for Mac Computers". The Amazon link is:
Having a minimum of 2GB of RAM per core is recommended, yes. You don't say how many you have, but depending on the number of corse, YMMV. The effect will be greater the more cores you have. But I find it odd that the bg rendering of all things should show such an improvement, since that is done near exclusively on the GPU, i.e. in VRAM not in RAM.
As I wrote in both the subject and the opening paragraph, these were base model units, so 12GB RAM and a single 4 core Xeon and the bottom graphics.
I suspect that the speed is related to Mavericks not having to play the memory compression games as heavily because of the added breathing space, or maybe the Panasonic CODEC is more responsive with more RAM. Regardless, the background optimized media render of an 18 minute AVCHD clip went from ~9 minutes to ~6 minutes, and that result is reproducible.