What would be the optimal SAN speed?

Should we configure all SAN ports to work on the fastest possible speed or would configuring all ports to work at the same speed be the best practice?

I know, "it depends", but when random high latency shows up it's hard to find the actual slowest device on the whole SAN environment.

So set everything to work at 4 Gb, 8 Gb or should you even use 16 Gb where possible?

I'm thinking use 16 Gb whenever possible, for example between switches on the portchannels, so the limited number of ISLs can transport the data to the remote switch as fast as possible. But what about end devices? As fast as possible or the same speed as all other end devices?

I can think of a number of reasons why everything on the same speed can be the best practice, but I can also think of reasons why portchannels need to be as fast as possible.

Any comments on this topic?