No, you won't see sustained 500MB/s transfer speeds. That's not how things work. And SATA is a serial bus, so each drive has its own dedicated port / controller. Taking out the GT drive won't change anything, because they don't share bandwidth that way.
 
File transfers will depend on the OS handling them, the speeds of the source and destination drives, and the protocols used to make the transfer. So if we're talking about simply dragging files from one drive to another, 140MB/s is pretty darn good considering Windows is pretty slow at moving large numbers of files & folders around.
 
You also need to keep in mind that SandForce powered SSDs give faster performance with compressible data. If you're moving  movies, DVDs, music, jpgs, or zipped files around you won't get the absolute best performance either.
 
  If you want to see 500MB/s sequential file performance, then take the OS out of the equation, use compressible data, and use an equally fast or faster pair of drives.
 
Edit: I shall add that if you were using a spinning drive as the destination, then you were probably maxing out the speed of the platter drive. Some HDDs sold today can't even reach 140MB/s, let alone sustain it for long. As the HDD fills up the throughput will slow down like the slope of a hill. Google HD Tach, it will show you what I mean by this.