Regardless of your technological stance, the question should be framed in terms of your market. Are you going to be creating a game that runs on all available hardware? Or are you going to be the next Project Offset?
According to Valve DX11 capable hardware is only 5.6% of the market.
The other factors are - does your "nano" team have the bandwidth create the content and pipelines that take advantage of more recent hardware? Or are you going to be beg/borrowing/stealing art to ship something?
Note that Valve themselves support a huge range of hardware. They have a scalable engine, and the resources to do so.
Personally, I would concentrate on shipping something that makes money as soon as possible. Yes you will need to to refactor later, even if you design your engine well.
Remember your market - they point of the game is to sell. As a compromise, I would probably target DX10 as a minimum spec. You'd get around 50% if you shipped tomorrow, and the interface is a lot cleaner than DX9. I believe, but I'm not certain, that the API difference between DX10-11 is a lot smaller than DX9-10.