I imagine having a faster PB helps in negotiating future appearance fees at any marathon. If race directors and fans see a 2:08 pb they put that runner in a different category than a 2:12 runner who ran their pb in NYC. Running a fast course for your debut could help set you up for future racing even if you have better ultimate prospects on a tougher course like Boston or NYC.
I'll predict a 2:08 from him right now. He ran sub 61 half at the USATF championships on a quick course. Chicago is a fast course as long as the weather isn't absolutely terrible, it will probably be reasonable enough to not affect him much. And it's well known he is more of a strength guy than a high end speed guy, so his half marathon speed will likely hold up well stretching out to the full marathon. Rupp ran 2:06 last year at Chicago on a less than ideal weather day, I don't see Mantz being more than two minutes behind that. If the stars align and it's a great weather day and there are people running a consistent 2:06/2:07 pace I think he could hold on to a sub 2:08. Go ahead and call me crazy, but there are a lot of people who have run 2:10 at Chicago who were not on the level that Mantz should be.
I think Mantz runs a new American debut record of 2:07:53. He'll try to run with the lead pack but will fall back in no-man's land, will likely place out of the top 5 but still inside the top 10.