I think it will allow around 10 seconds exposure without star trails, but needed to get inputs form anyone who had used this setup before. I couldn't find any sample photos online. From the chart here http://petapixel.com/2014/01/29/picking-great-lens-milky-way-photography/ ,the F1.4 lens is acceptable (i.e. with score >1000 as per the author), but would the F1.8 be totally useless with regard to night shots?
Also I am assuming with the 50mm lens on the 2x crop sensor, I will still get the 50mm focal length (i.e. same as if I was using a 25mm MFT lens) but the FOV will be less. Please correct me if I am wrong, to me this means that the only downside is that I will have to stitch together many images taken separately.