Depends on your use of the subsystem. Max throughput is not the only use, the OP is mentioning small block I/O and X25-E's. With that (80/20 rule) gives about 26-29,000 IOPS at 4KiB each. Controllers generally cap out around 200,000-300,000 IOPS. Putting all 8 drives on on controller would come close to saturating it's capabilities (not to mention the sata issue of being uni-directional in it's data path). With that in mind, it's arguably better to get two controllers that are cheaper than wait on a new controller and still hit the same IOPS bottleneck. Spread the load. It really comes down to what your design goal is.