I've done this and plan to do more to supplement my retirement.  

I stumbled into real estate investing while looking into ways to make passive income to supplement my regular job. 

This means you cannot use the money in your own interest, so these accounts will likely be a good supplement to investing you do outside of retirement funds, not a replacement for it.

Would we not all supplement our good tenants rent by $200/month.If a landlord is operating a business in which costs rise annually why would they consider lowering their tenants rent when the market does not warrant it.

Plus hold money for property taxes plus the dreaded supplemental tax bill that will inevitably show up.

As for market rents, unlike many landlords I do not believe a well run business supplements tenants rent.

Eventually, I would like to supplement enough income to scale back to a 40-50hr work week at my job and break away from the turnkey side of it to further maximize returns. 

The main reason we'd like to get into RE is to help supplement our income since my wife stays home with our 3 young children.

Costar has good data, but you'll want to supplement with local research (apartments.com, craigslist, zillow, etc).

Supplementing your tenants rent is charity not business.