The difference between the SCSI U320 drive performance and bus speed. Say you have a SCSI U160 controller (160 MB/sec) and a U320 controller (320 MB/sec). You hook up your U320 drive up to the U160 controller and get roughly 72 MB/sec with hdparm (hdparm -t /dev/sda). Then you hook it up to the U320 controller and get exactly the same thing 72 MB/sec. Now your bus bandwidth has doubled but your speed is the same. 160 MB/sec and 320 MB/sec are the max bus speeds not the drive speeds. This means if I take 2 drives that are running at roughly 72 MB/sec and max them out it would be about 140 MB/sec on the bus. A U160 controllers bus can handle this but throw another drive on the bus and your up to 210 MB/sec. Way over the 160 MB/sec max. The bus becomes saturated and performace takes a big hit. But put these 3 drives on a U320 bus and you have bandwidth to spare. You could run 4 drives that do 72 MB/sec and still not max out the bus and degrade performance. U160 and U320 were created so you could put more disks on the bus and not saturate it if they were all accessing at once.
So the numbers U160 and U320 don't mean the drives are going any faster. The drives are limited by many things like rotational latency, data clock rate, cache speed/size, seek time, etc.
Other good things to remember are if you use two or more different devices on a SCSI bus your devices will work at the bus speed of the slowest devices bus speed. So if you have U320 bus and 3 U320 drives and one U160 drive on that bus then your bus will be limited to U160 speeds. The devices negotiate at a speed they both have in common. Also, if your not seeing bus speeds that look right you have to make sure all of you hardware can support the speeds of the devices your using. The controller, cables, backplane, and terminator have to support the bus speeds your devices are trying to use.