After changing over the files & lightroom to the very fast RAID 5 HD, lightroom runs the same.
Same processing, metadata, previews etc...same, same, same.
How are you evaluating the speed? Export isn't I/O limited, so I wouldn't expect to see any difference. Preview generation is again, processor intensive, not I/O intensive. Where I would expect to see speed improvements would be in updating XMP metadata for say, several hundred images or in reading in new metadata for modified images. Those are the tasks that are I/O limited.
Also note that the data blocks that are generally thought to be "bigger is better" for Photoshop scratch disk and large image use (and video) is actually the opposite for databases where small block is better/faster. I tend to split the differences.
Also, as said, RAID 5 isn't known for it's pure, raw speed. RAID 0, just stripped, no parity, would be a lot faster...
When I set up my "monster machine" I made the decision to forgo parity (and a degree of safety) by using a 3 drive, internal stripped (RAID 0) internal for working files. Because of the risk of the stripped drive failure I also do automated, nightly backups via Retrospect to a mirrored external array. That way, I only really risk one day's work and have a two dupe backup externally. Then, from time to time I'll backup the external drives to my NAS RAID 5 networked drive...
If you want to speed up Lightroom (and Photoshop) the order of importance is;
1st-CPU speed, the more cores, the better (well, up to say a max of dual, dual core. quad cores are currently memory starved for big app use like Photoshop)
2nd-Ram. While Lightroom is still a 32 bit app and thus can't address more than 4 gigs max (actually I think Lightroom currently is limited to 3 gigs max), part of the issue with ram is system paging when the system runs low of free ram (including situations where the free ram is fragmented). And, unless you re running XP or Vista 64 bit, or Leopard, there's really not a compelling story for gong beyond 4-6 gigs. The Mac Pro towers and Vista 64 CAN make good use of more ram, but until we get 64 bit apps, it won't help apps s much as it will help the system and running multiple apps.
3rd-Disk I/O. The final potential speedup is disk I/O. This will also provide the least bang for the buck however...at best you'll be looking at maybe 5-10% improvements in those functions that are I/O limited...that said, Photoshop, running with it's scratch disk on a really fast RAID 0 stripped multiple array WILL improve Photoshop performance in those situations where you are working on really big images or lots of images open at once. Adobe has done some speed tests for Photoshop, see: [a href=\"http://blogs.adobe.com/scottbyer/2007/09/photoshop_world.html]Photoshop World - Heavy Lifting[/url] written by Photoshop engineer Scott Byer.