Is there any chance you guys could do some sort of benchmarks comparing the different hard drives proposed to the stock drives? I'd be interested in seeing how much of a difference it would make, especially for comparable 5.4k rpm drives with large cache. I've got several spare WD blacks laying around, wondering if it's worth swapping.
Several benchmarks exist. The results are as you would expect, bigger 5400RPM drives help a bit, rare 7200RPM drives help a bit more, SSDs help a lot more.
It varies game to game, some games cut huge chunks of the loading time, others less so because they were never slow at loading in the first place.
If you're playing Fallout constantly on PS4 which has damn near 1 minute loads, then an SSD can halve that. If you're playing Gran Turismo with 15 second loads, then it doesn't really matter in the first place.
I always found the idea of increasing internal size by swapping in a $100 1TB SSD a bit perverse, unless the console was my only source of gaming goodness. The console is cheap, a 1TB drive alone costing half as much as the entire machine.
Best bet for me was either using a cheap $50 500GB SSD externally to store the slow loading games on, or going for a 2TB mechanical internal to boost size and improve loading a decent amount.