Thunderbolt 3 & eGPU viable for Video Processing?
I've looked into this recently myself, given that the MBP line is not exactly a workhorse when it comes to GPU processing. 

eGPUs are actually being done now over TB2, with great success. I have a friend who purchased a similar unit to the Akitio Thunder2 (the Bizon-Tech version), put a decent GPU in it (I think it was the GTX960 or similar), and experienced almost 50% faster render time in Premiere (the latest versions of CC support GPU acceleration for encode/decode with some codecs). 

The math gets a little funky, but actually this Tested article comparing PCI Express to Thunderbolt indicates that the Thunderbolt2 spec, in practice, provides almost the equivalent of a PCIe3.0 interface. The bandwidth speed lost will be minimal. Further, the article indicates that performance of the graphics card that is lost by a slower transfer speed is, for all practical intents and purposes, negligible (as little as 5% in most cases).

I'm certain that the performance gained by an eGPU box, even over TB2 on current-gen hardware, would vastly outstrip the current performance you're getting on a CPU-accellerated-only laptop. 

And, the best part is, you're out only around $1500 for a box and a good graphics card.

For the record:

I don't know what notebook you're using, but I'll assume a relatively recent MBP. I'm working here from a 2014 15" Retina which happens to have dual GPUs (integrated Intel Iris and discrete GeForce GT 750M), and based strictly on CUDA cores, you'll have to pick up something in the K1200+ range if you want to see a significant improvement (the GT750M has 384 cores). However, it's relatively weak at only 2GB GDDR5 (roughly comparable to a K620 in terms of cuda cores and memory).

Improving both specs is important, particularly for 4K or RAW video. I would recommend something like a K2200 or K5000, or if you've had a good year, any of the Maxwell-class cards would be a beautiful, beautiful thing.



TechInferno - 2015 MBP + GTX970 + Akitio Thunder2 via TB2

LinusTechTips - 2012 MBP + GTX760 + Akitio Thunder2 via TB1

Bizon-Tech - Akitio-based custom enclosure for high-wattage GPUs

A useful comparison of top-end Kepler vs Maxwell NVIDIA architecture

Anecdotally, in 2014 I saw a live demo in NVIDIA's booth at NAB of what the K6000 card is capable of - at the time, mindblowing performance. They put dual K6000's in a Z820 workstation and did realtime 4k debayering of RED Raw, with scale and blur effects, in Premiere. So yeah, duh - it's a powerful card, right? Point is, you don't have to look at the latest Maxwell cards to get performance improvements in an eGPU setup - even a single quality card will provide much more efficient and expedient results than CPU-only.