Am I worried about the warranty?? Absolutely!! I bought Apple Care that won't expire till 2017. The main reason for installing an internal HDD was speed. I take a lot of photos and was storing and working with them off a NAS which was way too slow. And there is no way I'm going to wait till my Apple Care expires in 2017 before doing something about it. I had briefly considered external storage but I like to keep my workspace as quiet as possible. And external enclosures without fans just burn themselves out prematurely. So I decided the best route is to install an HDD internally.
You are correct in assuming that removing and re-installing the LCD is the most difficult part. iFixit already provides a very thorough guide on removing the LCD. In a way, the 5K model's LCD is easier to remove as it only has two cables while the other iMacs models have four cables to remove and reattach. In removing the LCD, I just used a plastic pizza cutter from iFixit and proceeded to cut along the three sides. Once the adhesive is cut and I was able to lift the LCD and remove the two cables, the bottom is easy to remove. I've found an adhesive strip (about 1" long that was loose) on both sides and when I pulled it, the adhesive was removed cleanly across the bottom. Bonus!
Re-installing the LCD is tricky. I had applied the adhesive around the three sides of the frame (top and two sides) but did not remove the backing. The reason is that I want to make sure the drive is NOT DOA. None of the external enclosures nor drive docks support a 6TB drive so I have no way of testing to see if the drive is fully operational before I install it.
I applied the adhesive strips to the bottom of the LCD and ripped the backing off so it was ready to stick. I placed the LCD into position making sure to lay the LCD as flat with the iMac as possible minimizing the gap. The side to side alignment is also critical. And don't forget to reconnect the two cables before laying the LCD down on top of the iMac frame. Now I did NOT peel the backing off the three sides so only the bottom adhesive is holding the LCD to the iMac. I used two long strips of adhesive tape to temporarily hold the screen to the frame. After two days of intensive drive use, I remove the temporary tape from the top, pulled the screen forward by about ¼" and remove the adhesive backing on all three sides to seal the LCD permanently. Done!
Drive Mounting Pins:
If you look at this photo from iFixit, the HDD is held in place using four drive mounting pins (p/n 922-7001). I was lucky enough to find a set of two off eBay for $1.00 each. The seller turned out to be PowerBook Medic.
As I've mentioned before, the HDD mounting brackets were already installed in my 5K iMac. Remove the left speaker, remove the two screws off the left drive bracket and you're ready to mount the HDD. But before mounting the drive, be sure to install the HDD SATA cable.
The HDD has been in my iMac for several days now and I couldn't be happier. The entire procedure took about 1½ hours. Have fun!