heh, the P3700 400GB is around 1.2-1.5k$ and a single PCIe card would suffice. The writes for which you would get a SLOG are never cached. For full-on memory caching of all writes you could disable sync writes - but that doesn't gurantee you that the writes are committed to the disk array.
However since your system is likely to not have PCIe 3.0 speeds at a triple-channel memory interface, you will probably not see the full 250MB/s on the P3700.