I did quite a bit of reading before purchasing a RAID card (though it's never enough) and one fairly consistent thing I noticed is how hot people seemed to think they ran if they didn't have air blowing across them. When sitting in a rack mounted server, this isn't a problem. However, the Shuttle XPC Cube isn't exactly roomy. I was a bit concerned, so when I picked out my LSI 9211-8i, I also grabbed an SLF-30 from Enzotech.

I wasn't positive this would fit, but I knew it would be close, and one reviewer on Amazon was using it for exactly this same purpose with positive results. There's also a post here about getting the heatsink off and just doing some maintenance.

SLF-30 sitting next to the LSI card with it's original black passive heatsink.
A bit of a twist and the heatsink popped off. Mine had the same hard TIM as the unraid.net forum post above. You can see where I actually scuffed the ink a bit removing it with a plastic spudger.
Popped the new heatsink on.
It fits -nearly- perfect. You can tell the push pins bend outwards just a bit, indicating that they could use another mm or two of room, but nothing that will cause problems.
The clearance between the LSI (left) and my NIC (right). They sit VERY close, thus the reason I was worried about airflow.
Not the most precise way to measure, but good enough. I didn't see a reading over 108 F (42 C) during operation. The manual specifies a max temp of 130 F (54 C).