Other factors of condition, schools, and rental increases are some of the other factors to consider that will not appear on a spreadsheet.

Second, your tax prep fees would increase dramatically.

This doesn't mean you should be looking at markets such as Detroit and Memphis, because even though it looks good on paper, if they are shady neighborhoods, you are going to spend a considerable amount more on property preservation (broken windows, stolen HVACs, etc.).The easiest way to increase cash flow would be on your existing property.

Refi'ing into a 15yr loan is likely to both increase your effective rate and crimp your future buying power by increasing your DTI. 

Has anyone joined & had success since the double price increase?

I would tithe on my increase, which would be profit whether you take it home or leave it in the business.

The market was increasing and we were able to find an off-market deal from some friends who held the property for over a year had no interest in fixing it up.

I had a less expensive one and it did increase, but not a lot. 

By being open to single family homes as well it really increases your choices, and for us, reduced the cost per door of acquiring our properties.  

I don't have an answer for your question, about increasing "winter" bookings, and I hope you've found one by now.