My cheapo iMac plan for photography

There's nothing simple about taking the display off a 27" Retina iMac; breaking the cover glass is a real risk and if you don't break the glass, getting it properly resealed on the computer is a challenge in itself.
You can simply use USB 3.0 to connect the SSD with effectively no performance difference, or spend more and get one that connects via Thunderbolt and have no performance difference as compared to installing the SSD internally.

 

I fully agree with your conclusion. Far better to go external. USB3 for a single drive (as TBolt is no faster) and then if RAID 0, 10 or 5,6 is used, then Tbolt makes more sense. I admit however if it were me doing the photo work, I would want the best processor possible (along with 16-32 gigs RAM). I base the latter on my own experience and certainly various sites will go even further than this for an ideal hobbyist system.

 

Well if you want to give it a go then here is the ifixit guide it will show what you face and explain the dangers.

https://www.ifixit.com/Guide/iMac+Intel+27-Inch+Retina+5K+Display+SSD+Replacement/30537

 

OP wrote:
"The conversion seems simple enough, but my concern is about breaking the screen even if the proper tools are used to open it. Is that a serious risk?"

Yes, it most certainly IS risky.
There are numerous reports from folks who "thought they could do the job", opened it up, and then... broke something inside.

There's a better way:
Get a USB3 external SSD of sufficient capacity.
Just plug it into a USB port and use it that way.
You can even make it into an "external booter" if you wish. The iMac will run great that way.

Booting and running this way (or just using the drive as "storage") will give you 85% of the speed (or a little more) than you'd get by having the drive installed inside.
Yes, it's a bit slower (but not much).
The upside is you DON'T break anything by opening up the iMac.

Take care:
If you buy a "bare" SSD and an external enclosure, MAKE SURE the enclosure you buy is SPECIFICALLY STATED to support UASP.
Or… buy a "ready to go" SSD like this:
http://www.amazon.com/exec/obidos/ASIN/B00ZTRY532?tag=delt-20