Quick progress update here. This Tuesday I went for the rails 4.1 upgrade on top of spree 2.1. The build looks good and I made some progress fixing some initial issues.
To do this I relaxed the rails 4.0 requirement in spree 2.1 so that we could bump it to rails 4.1 in OFN.
The main point is that I realized we don’t need to do/finish “bye bye spree” BEFORE the upgrade to rails 4.2. We can go for the rails 4.1 and rails 4.2 upgrade without moving all spree code to our side. We can move the spree code to OFN gradually and meanwhile get the rails upgrades done 
This is much better because of the time to market (bye bye spree estimate is 6 weeks), this means we are probably 3 dev weeks away from rails 4.1 and another 4 weeks away from rails 4.2.