The same considerations apply for Splunk as for relational databases. It depends how you're planning to use it. If you are performing rare searches, a slightly smaller block size may improve performance, because you're not transferring as much data. If you're performing dense searches and reports, you may read fewer blocks, though this effect is probably not large as the block reads are likely to be sequential.
Either way though, with filesystem block sizes in the normal range of 4kb to 64kb and with modern disk sizes and disk transfer times, this is unlikely to make a measurable difference.
