These are my USB 3.0 Speeds I get.
2 by 4TB (8TB) Seagate Backup Plus w/ Barracuda 7200rpm drives inside running through Belkin USB 3.0 cables to a Kanex 4 port USB 3.0 hub set up as RAID-0 through OS X Apple RAID.
233/264 write/read
1 3TB Seagate Backup Plus w/ Barracuda 7200rpm drives inside running through Belkin USB 3.0 cables to a Kanex 4 port USB 3.0 hub (No RAID)
192/201 write/read
And for comparison to the 8TB USB 3.0 RAID:
2 by 4TB (8TB) Barracuda 7200rpm drives running as RAID-0 in an OWC Mercury Elite Pro Dual (Oxford 846 chip) through eSata via a Lacie T-Bolt eSata Hub.
221/230 write/read
If I run the Apple RAID setup directly to the Mac mini via two dedicated Belkin cables (not through a Hub) I get a few more MB/s, but the Hub is more convenient for the slight loss in speed.
Couple of point I have noticed:
Quality cables can make a difference. I noticed someone on page 1 on this post is using the identical 3TB drive I have but is getting 25% less in speed to me. The only difference I can see is I have thrown the stock cable and used the belkin ones. Also I tried 3 metre cables and got far far slower speeds. My current cables are these:
http://store.apple.com/au/product/HA942ZM/A/belkin-micro-usb-30-cable-1m
My Kanex hub is this one:
http://www.kanexlive.com/usb3hub4x
Also the Seagate Backup Plus 4TB drive do not all have 7200rpm drive in them now unfortunately. Seagate has started putting 5900rpm 3 platter drives in them I believe (from searching the web). If you want the 7200 you need to make sure the box it comes in is the one with the Win 7 logo on it and you can see a Win 8 sticker has been put over the old Win 7 label on the side. Apparently the 5900rpm drives have a Windows 8 logo and no sticker. In other words you need the slightly older drives.
From what I have seen (buying 4 of them over the last 6 months), the 3TBs are all 7200rpm.
For those with slow speeds, you lose a massive amount of speed by running low rpm (5400) energy saving USB powered portable drives. If you need portable that's fine, but if you want capacity and speed make sure you get 7200rpm drives.
Also as John Kotches said, if you start using RAID-0 you need a backup plan as the drives are pushed to their limits and if one fails the array goes down. You can replace the broken drive but you have to rebuild the array so you lose all data.
Personally though, if i was going to buy a 4TB drive, I would buy 2 by 2TB 7200rpm drives and RAID-0 them. You get 4TB in volume with massive speed. If one of the 2TB drives goes you lose your data but you keep a spare 2TB. If you buy a single 4TB and it goes down, you lose the data and the drive. Obviously you lose the portability with RAID and you take up two ports and potentially a touch more space (unless you stack them).
Anyway, that is my 2c