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

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.