Help: Fusion Drive vs. Thunderbolt HDD for Huge Weddings

Help me understand which would be better for my workflow, just sticking with the Fusion Drive, or employing the LaCie Little Big Disk.

I shoot around 40 weddings a year, each wedding is around 30GB.

I keep one wedding on my Mac and one wedding in LR at a time. That wedding will stay there until it's finished. But on Saturdays, when I shoot a new wedding, that new wedding temporarily comes into LR for culling and a sneak peek. When I finish the sneak peek for my bride, I save the XMP files and then remove the new wedding from Lightroom.

My plan is to keep my main wedding on the Fusion Drive in Lightroom, and at 30GB, I assume the whole experience will be blazing fast, since the Fusion Drive is offering 400MB/S +

But my question is this : should I keep my other unedited weddings on a LaCie Little Big Disk? Or on the 5400RPM HDD inside the iMac? The LaCie offers read/write times around 200MB/S via Thunderbolt. But apparently that's only true for sequential transfers. Random access is more like 1.5MB/S. From what I understand, this is true of all HDD Raids. If you want fast random reads you simply cannot use an HDD.