In your playback settings, turn on "warn when frames are dropped due to hard disk performance." If that's giving warnings when you're getting the lagging, then yes, a faster drive would help.
But I'm betting that if you're applying slice x stuff, then it's just the GPU that isn't keeping up. And probably never will. Render it. Also try using the "better performance" playback setting when roughing things out. You'll get a lot more RT that way and less dropped frames.
[Jacob Brown]"i experience some lagging on playback within my FCPX timeline when there are composites or complicated effects like from SliceX.
That sounds more like you require more computational power than you have. Effects and composting require RAM and GPU power as well as fast drives. I personally find the G-Raids to run about about 300MBps over Thunderbolt, but less in the LaCie D2 drive, the D2 is an single drive whereas the G-Raid is 2 disc's mounted as single volume and as a spinning disc you are limited well below that for speed.
As a note with newer technologies, "cheaper" is often akin to "disposable" when talking about hardware.
You will get better drive performance from mulit disc arrays or from a single solid state volume than you can from a single spinning disc.
Post and Production Workflow Consultant
Production and Post Stereographer