The teardowns we have see so far have been for the 21.5" only. There is always the posibility that the 27" uses a different method to remove the screen.
You have a long chain going from your SSD to the system. And the systems bottleneck will always be at the slowest point.
SSD (SATA 2 or 3 im guessing), enclosure (is it SATA 2 or 3?), thunderbolt cable, thunderbolt connector, iMac.
Even if any of those factors are SATA 2 you will not see a large difference in overall speed. It will still be faster than a normal internal 5600 rpm drive, but will come at a cost. Looks to be about $100+ tax for the enclosure, and another $50 for the Thunderbolt cable (not included). I would recommend just spending the extra $100 and getting the fusion drive and then disabaling the software and using the drives seperatly.
i have special discounts for apple products because my friend is a reseller, but, of course, only for not customized products. 150€ its nothing compared to buying a customized imac. anyway, sata2 and sata3 changes a lot...if the seagate enclosure isnt sata3 then ill buy an usb 3.0...lets compare them:
TB solution: sata2 (3gpbs) + TB enclosure (10gbps) = 3gbps. Price = 150€
USB 3.0 solution: sata3 (6gbps) + USB3.0 enclosure (5gbps) = 5gbps. Price = 25€
howewer i prefer spending even 200€ and have sata3+tb=6gbps
If you have a discount then go with the base model.
Regardless of the best case scenario math, you will never reach these numbers. You will still be better off using the thunderbolt system. But for the cost of the USB3 system you could most likly put together two full systems.
What is the size of type of SSD you are using?