drdb (or AoE or iSCSI really) are great over local networks.  latency is usually pretty low and they are all low-overhead.

reading  this post, he is just putting the xfs journal on drdb.  All this really does is save the disk heads for doing some seeks which will lower the average latency of the drive somewhat because there is less travel(some sites online say up to 30% write improvement but that was surely a special workload).  You could do this to a local drive also.  Since this is just the journal, the only concern here is having the system go down and losing the journal device at the same time.

FYI, you can put the journal on a flash key as well as the journal doesnt take that much space.  the journal gets overwritten once the filesystem confirms the write so you really only have to store as much data as the filesystem has queue up to write.  really a 1GB journal is way overkill but a 1GB flash drive is cheap.  Also, the journal doesnt have to be smoking fast so a flashdrive that is readyboost capable(only a measurement of speed) should be fine.

I think that you have to make the journal device during filesystem creation but google might tell you that you can add an external journal later, im not 100% sure on this.