Have one at work. Never toyed with funplug or Alt-F, but I'm pretty sure we get better than 15 MB/s on backups, but not spectacular speeds. Could also be the HDD limiting the read/write speeds. We use WinRAR for the backup, and it travels over a GigE LAN including a DIR-655 to our Dell computers.
What type of internal HD are you using? We're using two WD Red's in RAID 1. I agree that RAID 5 wouldn't be your best option; you want to use that USB port as little as possible, maybe just for scheduled backups if they're small enough.