What people are missing I think here is that the bigger battery will unleash better performance. They will probably use the 335i (now 340i) as a benchmark and try to beat it.
340i starting price: $47,900. 0 to 60 4.8 sec
That would make sense for tesla to charge a big premium for that, especially to not canibalize the model s.. so I think anything under $10k overly optimistic.
Assuming your talking about driving performance only, that is not necessarily true. The bigger battery will enable better driving performance only if the power output from the battery is the limiting factor. Using the Model S for example, that is the case for dual motor vehicle, where you see jumps in performance for various battery configurations on the dual motor car. However, if you look at 0-60 times for the S60, S70, and S85, they generally hover around 5.4 - 5.5 sec, right? In this case, I presume its mostly the single large rear motor that is the limiting factor, and a larger battery won't change that. (Now, larger battery will improve charging performance, which I also find valuable.)
To relate back to your BMW analogy, I expect the larger battery plus dual motor to be about a $10K upgrade total (ex. $6.5K for battery and $3.5K for AWD). That probably gets the Model 3 0-60s into the mid-upper 4 second range.