I just wrote him back and said hey we get into contract I can run some of this stuff down but not going to spend hours figuring out all the fine details on all the components of a new build ..

I think assumptions are easy to make due to this being all text, but my mind is absolutely not made up which is why I am posting. i raised the concerns with my sponsors around the high IRR and fees and other points you all made and she countered with the air bnb component being the reason.

Built-in Roth components, spousal contributions, and an additional tax exemption also apply to the Solo 401k.

Depreciation is finite- with each component you accellerate, you will empty the bucket for future depreciation of that component. 

And don't forget the other component of real estate investing you lose in an exit - depreciation.  

For newly constructed, purchased or renovated properties and also retroactive generally over the last 10 years, building components are properly classified into individual units of property and accurate recovery periods for computing depreciation deductions.

The primary goal of cost segregation is to identify components that are not real property and therefore can be depreciated faster (typically over five, seven, and 15 years).

With older buildings, we need to be careful with the components of the building, such as roof, driveways, mechanicals.If you have to change the roof in the next 5 years, there goes any cash flow.

Does anyone have any actual data on the return on this component (previous appraisals, paired sales analysis, comparison on new construction with/without rooftop decks)? 

I think, in general, unless you're an extremely high W2 earner just looking to get into real estate for tax purposes, buying for cash flow gives you the durability to weather market fluctuations as you're never in a position where your back is against the wall and you need to sell... unless you have debt coming due.Now, that being said, if the true cash flow, after allocating for all expenses, vacancy, maintenance, capex, etc. is in the black, then I think taking a 5% CoC return in an appreciating market isn't a bad move, especially if there is a component of value-add.