Honestly I don't think the math works out on having sold your BTO Mac Pro for an older model you'll have to upgrade and then want to upgrade again down the road, because you're locked into kludging modern I/O and the processors are not going to get any faster. But it's your cash.
To equal or beat the nMP's SSD speed you'll need a PCIe card with striped SSDs. Sonnet, Apricorn, and OWC make options; I leave it to someone who actually chases that kind of speed to give you better tips on what the best current options are. Presumably you'd want additional storage as scratch disks for your Photoshop, After Effects, and Premiere work.
Adobe applications can use CUDA or OpenCL for rendering, I've tried using both on cards that supported it and haven't really ever noticed a substantial difference. CUDA is only necessary in Adobe AE with the raytracing, which was introduced and then de facto dead in less than two years, so it's pointless to worry about it (even with CUDA it was always dog-slow, so getting into Cinema or 3D plugins like Element is a better use of your time and money.) If you're fine dealing with no boot screens a modern Nvidia card is probably a better buy, as you need a card to fit the Mac Pro's power offerings.
To equal or beat the nMP's GPUs you'll need a AMD 7970 or better.
To equal or beat the nMP's SSD speed you'll need a PCIe card with striped SSDs. Sonnet, Apricorn, and OWC make options; I leave it to someone who actually chases that kind of speed to give you better tips on what the best current options are. Presumably you'd want additional storage as scratch disks for your Photoshop, After Effects, and Premiere work.
Adobe applications can use CUDA or OpenCL for rendering, I've tried using both on cards that supported it and haven't really ever noticed a substantial difference. CUDA is only necessary in Adobe AE with the raytracing, which was introduced and then de facto dead in less than two years, so it's pointless to worry about it (even with CUDA it was always dog-slow, so getting into Cinema or 3D plugins like Element is a better use of your time and money.) If you're fine dealing with no boot screens a modern Nvidia card is probably a better buy, as you need a card to fit the Mac Pro's power offerings.
To equal or beat the nMP's GPUs you'll need a AMD 7970 or better.