Replacing HDD of 5K iMac Fusion Drive

Lets first review what it is: Fusion Drive, simply put the SSD is a read/write buffer for the HDD.

So there is a limit of benefit with the size of the SSD and the cost of the SSD. Unlike HDD's SSD are still quite expensive! 4 to 5 cents per gigabyte for the hard drive and 25 cents per gigabyte for the SSD SSD vs. HDD: What's the Difference? So Apple tried to find the best SSD size for the given HDD they put in. I do think they should have continued using the larger SSD! But, they down sized! Apple slims 1 TB Fusion Drive down to a measly 24 GB of flash storage

So... Getting back to your Q's

If you need deeper storage then upgrading the HHD with a still larger HDD is possible. If you are looking for speed then swapping out the HDD with a SSD won't get you that much as the limit of the SATA III (6.0 Gb/s) interface is the limit here. As you have the older Fusion Drive model with the custom blade 128 GB SSD you're already hitting the max of the combined I/O channel as the SSD is doing its job of caching the reads & writes.

If we look at your systems SSD's PCIe slot 8.0 GT/s PCIe x4 its dam fast! So upgrading the blade SSD would be the best investment. And instead of configuring the system in a Fusion Drive config set it up as a dual discreet drive config so you are using each drive to the max for what they are best at.

Sorry to say you can't really RAID the drives in your system as the I/O speed difference will hold you back as the fastest you can go would be the slower of the two I/O's in this case the SATA 6 Gb/s Vs the PCI'e I/O 9 Gb/s.

Remember! Fusion Drives are only intended to be used across a HDD and SSD drive pair to improve the HDD's speed at a lower cost. Just like a SSHD is able to do the same thing within one drive.

The alternate option here is to just get an external Thunderbolt2 RAID drive (SSD) here we can push the I/O higher than the internal SATA ports limit as we can setup a RAID 0 config getting about 1 1/2 times (9 Gb/s) the throughput with 2 drives and 1 2/3rds with four drives (10 Gb/s) which is the limit of the Thunderbolt I/O within the system (directionally).

To put this into perspective: I have an older iMac I have a dual drive config 512 GB SSD and a 1 TB HDD which in my system is bottlenecked at SATA III (6.0 Gb/s). I also have a souped up MacPro (trashcan) with an external RAID'ed SSD which is connected with Thunderbolt2 RAID with 4 SSD drives. And I can tell you it screams!

So the bottomline here is break the Fusion Drive set and find the largest Apple blade SSD you can afford and put that in. Then migrate your OS and users accounts over to it and use your HDD for the deep storage. Or, get an external RAID'ed TB2 SSD drive migrating to it instead.