How is a modern iMac going to get bottlenecked on photo storage?
If you buy a basic iMac, it comes with the Fusion HDD, so as soon as your OS + image databases are larger than the SSD portion, you're back to the performance I/O limitation of a single platter spinning disk.
Thunderbolt evens the playing field for any reasonable "part-time"/"amateur" storage subsystem requirements. Same thing with the rest of the Mac line up.
True, within reason...which is also why I mentioned the YMMV here based on how responsive you want your system to be and how large of an image repository you have.
The memory throughput and DMI links are clearly faster than 10Gb/s. The storage subsystem can top out close to 10Gb/s. So where is this bottleneck?
Simplistically, any time that the intended workflow has to fall back to a single spindle HDD. Since the iMac's OEM drive isn't an SSD, nor is it really user-servicable, this factor may be an initial purchase factor/consideration.
how much data storage...
Which has little to nothing to do with which Mac model the person is using at the core of their set up. There is no single storage architecture imposed on any Mac user at this point with the modern models.
Sorry you missed the point: the point is that if the user's plan is to employ the iMac's internal storage as their primary data repository, then they have to trade-off the Fusion drive vs the varous sized SSDs .. and their respective costs .. at the planned time of initial purchase. True, a factor that can adjust this is what their plan is what they want to do if/when they outgrow that basic capability ... but this decision needs to be done cognizant with what that plan is going to be, and to what degree the Mac that they're looking at is compatible with user upgradability (and how).
And GRANTED, this is also going to be somewhat of a moot point once the new Mac Pro ships ... but it hasn't shipped yet, and the current (old) Mac Pro is still an option for the OP.
And case in point through a personal example: I've learned that single spindle HDDs are too frustratingly slow for my workflow requirements, which pragmatically necessitates either a RAID0 or an SSD. As of even today,
none of Apple's iMacs in OEM form offer internal SSDs large enough for my present library...at any price ... which forces me into an external solution for it on the outset, regardless of if I choose a RAID0 or SSD, and which protocol (TB or USB3) I choose to put it on.
FWIW, all of the above is merely hypothetical for me ... I did this research last year and ended up choosing to get a 2012 Mac Pro last year and took care of my data repository needs (12TB) internally. At the time, this approach was more cost-effective for my use case vs the iMac;
YMMV and not any BS "Scare Tactics" whatsoever: just sharing what I learned for my own application.
-hh