Talking from a 1.7.10 stand point but I think still applies:
I have ran a heavily modified version of Minecraft on various systems.
I play with a server so a lot of the overhead is offset onto another machine but I can say that while more ram helps (when you configure java to use more) the biggest culprit is your CPU/GFX combination. I have an older system with a 750gtx 250GB ssd and a socket 775 intel cpu and minecraft will run like hot butter, until I open a chest or some other task involving cpu cycles. That CPU grinds and churns frames into a stuttering mess.
Likewise I have a laptop with a an old radeon graphics chip, 1TB HDD, same ram amount, same java arguments but with an i5 cpu and Minecraft runs sluggish but steady in all cases.
Point being the ram didn’t make much difference other than allow for more mods.