2 TB3 ports with "4 PCIe lanes" - Do I get 8 in total or 4 across all connectors?
The Dell XPS 15 9575 has two Thunderbolt 3 ports and on Dell's website is says:
1. 2x USB-C 3.1 with Power Delivery & DisplayPort | 2. Headset jack | 3. Noble lock slot | 4. 2x Thunderbolt 3 with Power Delivery & DisplayPort (4 lanes of PCIe Gen 3) | 5. MicroSD card reader | 6. Battery gauge indicator
I know that it is very common for multiple USB ports to be hooked up to the same controller and thus sharing the bandwidth across all ports.
Does this also apply to Thunderbolt 3 or can I expect to get two full Thunderbolt 3 connectors with 4 PCIe Gen 3 lanes each?
Out of curiosity: Would it be possible to design an eGPU case that uses two TB3 ports to get an x8 connection for the GPU?
The link you posted says it uses 8th gen CPU which doesn't have integrated Thunderbolt. Therefore it's using discrete Thunderbolt. It's probably using a single controller with two ports so you're limited in total bandwidth to ≈2800 MB/s.
You cannot combine two Thunderbolt connections to get one x8 connection even if the Thunderbolt controller in the laptop was not limited to x4 upstream.
Laptops with integrated Thunderbolt (10th gen mobile CPU or later - Ice Lake and Tiger Lake) or Desktops with integrated Thunderbolt (Rocket Lake) should have max performance on at least any two ports (≈4700 MB/s - doing this from memory). I haven't seen Rocket Lake Thunderbolt benchmarks yet. I don't recall Tiger Lake benchmarks but maybe they exist in this forum somewhere. Itsage did Ice Lake benchmarks for sure. https://egpu.io/forums/laptop-computing/ice-lake-cpu-on-die-thunderbolt-3-controller-bandwidth/