It's easy to find endless articles about the performance of Cat5 vs Cat5e vs Cat6 but I'm curious about the real world performance. I frequently discover that clients of mine will have Cat5 in their homes and offices, but are able to operate at gig speeds anyway. Are folks likely to see a clear advantage in those situations from upgrading to Cat6?

For example, I have a client with a decent-sized network. About 30 workstations, several gigabit switches, two servers, etc. The servers aren't in the network closet with all the rest of the gear (not sure why, before my time), and are instead located about 50' away, connected by a Cat5 cable that's presumably longer than that. I'm tempted to suggest that they at least have me run a Cat6 line from the servers to the network closet. How much would that improve things?