Since you are using 50/34 drivetrain without problem, I can try it too. So I need exactly those;
And bolts & Ultegra 50/34 Chain rings or should be Rotor Q-rings?
And which bike do you have ?
In 2020 this bike was manufactured with 105 50/34, and when I look into crank set’s product details all same except BBs and pressfits. So I think if I have Ultegra R8000 Crank Set and FD with proper BB and pressfit. I’ld be fine,right? because it looks same…
I have a 3T exploro grx 2x
I am not sure about the ultegra bolts on rotor spider. best is to get both and test to be sure.
you have a gravel bike originally.
in my opinion is the bike geometry, your position, clothing , etc is what is slowing you down. not your drive train.
46/11 top gear at 90 rpm is 48.1 km/h that above your claimed 45+ km/h speeds!
Yeah, I remember Trek shipping out the first couple batches of Checkpoints with road groupsets. Again, you won’t have any issues with a 50/34 as long as you’re using the appropriate front derailleur. 
If the manufacturer says it will fit 50/34, then it will fit.
The bottom bracket is the same for GRX and Ultegra. They’re both just 24mm Hollowtech spindles, so no issues there either! 