not alot of mac users in this forum, so you may have to contact egpu manufactures or search the internet for other forums where people are using them, to get answers. there are several thunderbolt pcie or egpu manufactures, akitio, sonnet, and bizon are a few.
if you haven't already, you may want to try different playback resolutions, like 1/2 or 1/4. that should help with smoother playback. if you want better renders and/or export times, you will have to identify any bottlenecks. if the cpu is maxed, then there isn't much you can do. if the cpu isn't maxed then you need to identify if the gpu is being maxed, by monitoring the gpu usage. i don't use mac so i don't know which programs or system utilities monitor cpu and gpu usage in mac os.
if the gpu usage is over half and you think its the bottleneck, then perhaps an egpu will help. there may be some gpu performance loss from thunderbolt 2. the bizon egpu FAQ says not really, but this barefeats article suggests otherwise. you may not need to connect the video output of the egpu to another monitor or somehow re-route it to the imac 5k display, as premiere might be able to just use it as a compute card (setting mercury playback to cuda) and your display will work as it does now. you may not want to use any other external devices with the thunderbolt connection the egpu is using, to leave its full bandwidth for the gpu. i think the imac has two thunderbolt connections, so you could use the other for your bmd multidock.
hopefully this gets you on the right track, but again i don't use mac so i cannot confirm any of this.
Please please don't buy for imac 5k. I did this mistake and i'm really regret that. It's works like **** in everyway. I sell it half price.
Lot's of problems and Bizonbox webpage is full of ********. They didn't release my complaints their customer stories section.