I am wondering if its something to do with the oil breather system not working properly and oil is getting blown out of it and not going back into the sump. For it to use that much oil in 400 miles is not normal If it is not blowing out blue smoke on start up or when coming on boost and there is no signs of any major oil leaks that cuts out the usual things that you would think are causing it to have such a high oil consumption.I just did 370 miles in my 2wd Saff to Santa Pod in hot weather and some fast driving and when I checked the oil it needed less than half a pint to top it up again and my rebuilt engine has done about the same miles as yours.