I've been looking at the new 5K 27" iMac, and the RAM is user upgradeable. At least for this model go round they have not restricted you. Although good luck in finding any details about the differences between the four different video card options. So far I have had zero luck in finding any details, either online, or by going into an Apple store. I think I will be going with the i5 with the basic 8GB RAM, and then upgrade it to 24 GB, by fitting an additional 2×8 GB sticks. Although Apple only list the computers as supporting 32 GB RAM they will in fact support 64 GB, and there are suitable memory modules available, although adding 16 GB modules is not cheap, IIRC about £200 each when I looked. Personally I think i5 and 24 GB will be perfectly adequate. I was considering fitting my own SSD to it, but looking at the system I think I will get, the 250GB SSD is only an £80 upgrade, and a drive will cost that. As I understand it now, the drive upgrade is a lot more work than doing the RAM.
Alan