I would say, yes it makes a difference. Rendering in FCPX happens in tiles. The size of the tiles is determined by a combination of the amount of memory they consume, the speed of the graphics card, and other similar factors. Increasing the video RAM will allow more and/or bigger tiles to fit on the card at one time. Bigger tiles means fewer passes to make the same adjustments. More tiles means less trips back and forth to main memory or the disk to retrieve image data.
Whether that helps you or not depends on how you edit. If you're mainly just cutting HD clips together with no effects or color corrections, it might not make a big difference. But if you're editing 4k or 8k footage, adding 2D or 3D titles (particularly 3D), applying lots of effects and color corrections, doing retiming, using the 360° functionality, etc. then it will make a larger difference. In my experience, every little bit helps.