I can completely understand you wanting to run factory parts, some folks dont understand but in reality they generally have less parts to fail, less potential for leaks,less issues, are cleaner and it has that factory look that many like. Also the likely hood of a r151f rear case half section failing in zero to none. I have seen the back half sections sell for over $400 though. So it comes down to what it is worth to you. For me I almost always run the r151f transmission as a whole, just like now in my first gen.