The Mac Pro (4,1 & 5,1 I am assuming you are talking about) uses triple channel RAM, so the RAM gets its fastest speed if you put it in in banks of 3. However, the motherboard has two sets of 4 slots, so if you put in 4 modules rather than 3 into each bank, the RAM will run slightly slower - though probably not so much as you'd notice in the real world, at least in the way most people here use their machines.
What it will enable you to do is load far more samples into memory. I have always thought of RAM in this way:
The amount of RAM won't ever speed up your computer, but if you do not have enough of it, your computer will slow down, and sometimes dramatically so.
If you run low on RAM, the computer will start writing temp files to disk, which is an order of magnitude slower than have that data in RAM.