Would USB 3 make a difference for a USB 2 5400 RPM drive?

I've got a USB 2 5400 RPM external HD. I'm thinking of getting a new computer with USB 3 ports.

If I'm currently getting read/write speeds from the external HD on USB 2 of roughly 30 mb/s (both read and write), then would upgrading to USB 3 make a difference? Since USB 2 is roughly 480 mbps (60 mb/s), which is more than the 30 mb/s I'm getting now, then won't it technically make no difference at all?

Specifically, the drive I have is this very popular one.

The reason USB 2.0 is practically limited to roughly 50% if it's rated speed (20-30MB/s vs. 60MB/s) is that is not full-duplex, thus every data packet which comes to the host (PC) will need another packet to the device (HDD), effectively limiting the speed to ½ of the theoretical 480MBit/s.

Since USB 3.0 has not only much higher rated transfer speeds, but also allows full-duplex operations, you can expect to see almost maximum device speed (usually around 90-130MB/s).

You're already using that drive at its maximum performance.

USB 2.0 maximum throughput is 480 Mbps (lowercase 'b' - megabits per second) and you're getting 30 MBps out of it (uppercase 'B' - megabytes per second).

1 MB = 8 Mb, so USB 2.0's theoretical throughput is 60 MBps. But USB can't work with full duplex, and that means it's 60 MBps in both directions added together. That's where your 30 MBps comes from.

USB 3.0 is faster, but as you stated your drive only supports USB 2.0 (which is already used at 100% of it's performance). You won't get any performance improvement until you move the drive to a USB 3.0 case.

If I'm currently getting read/write speeds from the external HD on USB 2 of roughly 30 MB/s (both read and write)

30 MB/sec is a normal speed for USB 2.

It is not 60 MB/sec. Mostly because there is a lot of overhead in the USB protocol. And partially because and 10% of the bandwidth is reserved for USB 1 transactions.

would upgrading to USB 3 make a difference?

Yes, moving to a faster interface would increase the speed. It does not matter if this is USB 3 (much faster, but not present on all computers), eSATA (native link speed to the disk and no overhead), or something else.