[berntb]

>>"If you could choose n where your language is n times as slow and you are n times as productive with this language were both true which value of n would you choose?"

>>if I can build an app 5 times quicker than before, and the amount of time involved isn't trivial (i.e. a few hours) then then development time is vastly more important than the end speed of the code.

I would set n somewhere between 50 and 100. Then benchmark the resulting programs and rewrite bottlenecks in C. (Just think about it; years of productivity in a month! Then another few months to optimise the prototype.)