RTX 2080 is 215W vs GTX 1080Ti 250W
RTX 2070 175W vs GTX 1080 180W
RTX 2060 160W vs GTX 1070Ti 180W
Assuming MaxQ means minimum possible spec (which it mostly does), RTX 2080 MaxQ is 80W, GTX 1080 MaxQ is 90W and Desktop GTX 1080 is 180W.
Power is the main limitation in mobile.
From the Shadow of Mordor graph, 9.5% better at 12% power limit deficit is not a bad result.
First off, you're comparing different levels of card, so your comparison is invalid here. Secondly, the paper specs do not reflect real-world power consumption.
When TPU tested both cards, 2080Ti Founders consumed 18W more than the 1080Ti Founders while running Furmark, and the peak gaming load for 1080Ti was 267W versus 289W.
Secondly, we aren't discussing whether 2080Ti laptops will be faster than 1080Ti ones. We're discussing whether fitting a 2080Ti into a laptop requires more aggressive downclocking than fitting a 1080Ti into a laptop does.
Since the 2080Ti consumes more power, the simple answer is yes, it will require more aggressive downclocking, because while it is very easy to make a GPU cooler larger and better on desktop, the same cannot be said of increasing the size of cooler in a laptop.
The Desktop's thermal envelope can expand, and the laptop's stays the same. That means the performance delta also grows.