What is the clock speed of your processor? My suspicions are that you probably will not benefit significantly by increasing the RAM.
I have an Intel Core i5 3470 clocked at 3.2 GHz
I have a similar processor, but I'm only using 8 GB RAM. In talking with my son-in-law who is a computer technician, I was told to look at the performance monitor. He said that if only the processor performance maxed then adding more RAM wouldn't help. When I checked the performance monitor both the processor and the RAM maxed out. So he indicated that I could probably benefit some by adding more RAM. But he also indicated that I would see a greater performance boost with an i7 processor. I'm not ready to purchase a new computer right now. But I'm going to increase my RAM to 16 GB based on his recommendations.
I'm working with 24 megapixel images from a DSLR and have been able to stitch as many as 18 raw images (14 bit). But it takes Lightroom close to 5 min. to process the stitching. I will be interested to see if there is much difference after I increase the RAM. I'm only sharing what I have been told because I'm facing the same dilemma you are. The large panoramas can help produce much higher quality images. But I think I need a computer with a faster processor and more RAM if I expect any significant increase in processing speed.