Custom Fusion Setup

I'm looking to order a new iMac and want to get some opinions on the drive configuration.
Whilst I can see the merits of the fusion drive, I do feel that no option to specify the size of the ssd portion is disappointing. I use my computer for web development and can often have over 128gb of files in regular use if I am running MAMP, all my software, a large client database clone and the associated site files. At this point, the Apple Fusion drive is potentially going to slow down to HDD only speeds. In my opinion, the upgrade cost to 1TB SSD is excessive.
The though I have is to order with a 512GB SSD, and then add my new 3TB LaCie Thunderbolt 2 drive. Set them up as one core storage volume and configure data tiering. i.e. create a custom Fusion drive of 3.5TB
This will give me the speed benefits of SSD, a large buffer of storage space, and the advantage of automatic data management once the SSD starts to fill up, moving finished jobs back to the Thunderbolt drive, whilst bringing the latest jobs into SSD.

Is this a workable solution? Has anyone here done this and got it working correctly, i.e. NOT JUST as core storage. Does the BTO 512GB SSD have the same data transfer rates as the pci-e 128GB SSD that Apple use for the factory Fusion?
My other option is to go with factory Fusion, and if i am finding the machine sluggish once the warranty has expired, swap out the pci-e SSD for a larger size.
Whilst I could stretch to a 1TB SSD BTO, I can't see any massive gains over a 512GB SSD if i use the setup I mentioned above.

Give me your thoughts, opinions, and better still, real world experiences of this type of setup.

Thanks