I agree completely... I am going to add some additional information. This is for others that are reading on this subject jeahrens, since you seem to have some understanding on the subject.
"Real world" (whatever that means) data transfer rates for drive (based off of RPM speed, and knowing that RPM speed doesn't mean everything) has been shown on a couple of sites as follows:
5400 RPM --- 75MB/s
7200 RPM --- 100MB/s
10000 RPM --- 140MB/s
All fall well above the 54Mb/s (or 6.75 MB/s) minimum (and really, the current maximum *required*) blu-ray data transfer requirement.
And yes, as we both mention, if that data is striped (spread) across multiple drives, you should get even better hard drive throughput.
Another factors that improve hard drive performance: data density...
Take two 1TB hard drives with the same RPM speed. One has a single 1TB platter inside, and the other has (2) 500GB platters, the single 1TB platter will win in performance every time. This is just in data seek times since the data is crammed closer together on the single platter, but it will make a difference if multiple files are being read from the same disk.
So combine those together and a 5400RPM drive may work for your needs, but you can even go to YouTube and search people comparing and timing the difference in computing speed (boot ups, copies, shut downs) and see a big difference in performance by going to 7200RPM. But yes, for what we are trying to do here, it actually probably doesn't matter until you start sending multiple movies at full bitrate across your home gigabit network at the same time to different displays. I just know that with my setup I can do, what I mentioned earlier, or a full 100% data transmission to my HT, a reduced 24MB/s playback to my Roku, stream across our 102 Mb (small b)/s wireless, which always seems to be more like 70Mb/s in real use to my daughter's tablet, and I can get something on my phone via 4G (not sure what the server is transcoding the audio/video down to for the tablet or cell phone), all at the same time without a hiccup. I fear that with 5400 RPM drives I may have seen a "blip" here or there, and it wasn't worth the risk. I mean, I was going to go with 4TB 5400RPM drives for my build and instead went with more 3TB 7200RPM drives to get the space that I wanted. I am glad I did. Then again, it actually cost me less at about $100 ave per 3TB drive due to waiting for sales vs. what was about $180 per drive for the extra 1TB per drive in 5400RPM.
Some things NOT to get hung up on are SATA 2 (300MB/s) vs SATA 3 (600 MB/s). Currently SATA 2 can handle the data throughput of even some lightning fast 15,000 RPM drives that max out around 200MB/s. Of course, if you are buying new, go for SATA 3 (or whatever is the current and latest spec).
Another non-issue is cache. Back when drives had 1 or 2 MB of cache, it was a big deal, but getting above 8MB and you won't see much, if any improvement. Tom's Hardware did a test on disks with different cache sizes a few years back. It compared 8MB vs 16MB cache, and the difference was 2.1% faster, which is just over 1 second for every minute. Other go on to say that it is even less increase when the cache gets above 32MB, and almost pointless for anyone accessing random, rarely used, very large files (like a movie RIP), since the cache has only frequently stored data in it.
Anyway, we've talked drives almost to death. I think that we both still recommend going 4-bay, with 3TB 7200RPM drives.