The Built-In dGPU in the 15" is built for power efficiency more than anything else and while it isn't bad, per se, it is never going to approach the higher tier NVIDIA cards.
If you need/want 1070/1080 performance, you need to be looking at a Vega 64, not an RX580, plus the cost of the eGPU enclosure.
The cost of a nicely equipped 13" i7/16GB/512GB plus the eGPU(Razer Core X) and an 8GB Vega64 is going to be around $3399.00 USD total ($2499.00-MBP, $299.00-Razer Core X and $599.00-SapphireTech Nitro+ Vega 64 LE).
On the flip side, for $3499, you can max out the CPU and RAM on a 15" or for $3599, max out the RAM, keep the base CPU and move to 1TB of storage. The benefit being that you have a 6-core 15" model with 32GB of RAM, which is no slouch and you can add an eGPU and a 7nm Vega "Navi" in mid-2019.
Personally, I want all the firepower I can get and I would opt for the 15" w/ 32GB of DRAM and the 1TB Flash Storage. I would also get the Core i9 upgrade, but that's me.
Here is hoping Apple upgrade their offerings on the BlackMagic enclosure to include Vega64 in the future.
Of course I could just get the Razer Core, it is just a pain that should anything go wrong, I have to ship it internationally...