Just wondering why you are not using the Thunderbolt ports for fast storage? The Photoshop scratch disk should equal the speed of your internal drive. Since it is PCI SSD, you will create a bottle neck by using your external USB connected drives for a scratch disk. In your case, it would be better to leave the Photoshop scratch disk on the internal drive. Both Aperture and Lightroom would benefit having their libraries stored on a faster drive.
Investing in Thunderbolt2 peripherals would help keep your new MacBook Pro current and up to speed.
http://eshop.macsales.com/shop/Thunderbolt/PCIe_Chassis/Mercury_Helios_Accelsior_E2/USB 3.0 5GB/sec max transfer rate
Thunderbolt 10GB/sec
Thunderbolt2 20GB/sec
Of course actual speeds will be much slower than the maximum transfer rates and are dependent on the media SSD (SATA, PCI ) Hard Drives are dependent on individual speeds. Running external drives in RAID configuration increases their transfer rates tremendously but RAID has it's drawbacks. RAID 0 gives you the fastest transfer rates but a drive failure will result in data loss, RAID 5 gives you the benefits of RAID 0 with the advantage of being able to recover the information should one drive fail. It has the disadvantage of losing the total capacity of one of the drives in the RAID array. (4 x 2TB = 8TB RAID 0) (4 x 2TB = 6TB RAID 5). An external RAID device with its own RAID controller built in will be more efficient than using software RAID built in OSX.
I am running a 3TB Seagate Backup Plus drive (Thunderbolt 1) on my Thunderbolt2 equipped 2013 MacPro transfer rates are about 195MB/sec
The Apple PCI SSD gives me transfer rates of about 1,061MB/sec
Promise Pegasus2 R4 (Thunderbolt 2) (4 x 2TB Toshiba HD) Running in RAID 5 produces transfer rates of 657MB/sec
*Scores were computed using Blackmagic Disk Speed Test