To me that means figuring out where I want to be and then making bench mark goals toward the place that I want to end up. 

In general, is there some benchmark on how much is a house on pie shaped lot lower than a regular rectangular lot?

*Translation: the T10 is the 10yr Treasury rate which is the reference benchmark for pretty much everything real estate related whether debt or equity.

If I'm benchmarking this investment against the S&P 500 which returns about 9% annually with dividends reinvested over the past 30-40 years, would IRR be the correct tool to use?

So you are using loss to lease in an operational capacity as a benchmark to where you would like to be. 

If I can get purchase, closing costs, holding costs, etc, under that $90k benchmark, they will immediately refinance so I can pull 100% of the money in the deal back out.  

It's usually a little tough to get information on individual room rentals so I would suggest you look at what similar style studios are renting for in your market area and use that as the benchmark for the maximum price that a bedroom would rent for.

It's smart to bench mark values throughout ownership.

I finally bought my first new couch from a store after meeting my first big benchmark of $1MM in RE :) Not too expensive, but It's great!

Especially since my benchmark is a nice, low-maintenance S&P 500 fund that has historically returned an average of 12% yearly over the long term.If you're in a better area jump in!