I just recently replaced the cylinder head/head gasket/injectors on my R52 (non S). Had some issues with carbon fouling, coolant loss, and overheating, and decided to just redo everything.
Everything went pretty smooth. The only concern I had was that after removing the injectors, the old injectors had 4 nozzles on the bottom, and the new ones had 6 in a hex pattern. I purchased these injectors on ebay with the description ("4 x Flow Matched Fuel Injectors for Mini Cooper R50 R52 0280155991 13531487607") Which seem to be the correct part numbers for the non-S models. ( I purchased these 2 years ago, so going back for more research is not a great option).

The car is running great, engine is purring, and has more power than it did before. But it throws a code on the #2 O2 sensor (after cat), and never did before. Thinking it could be a fluke of the engine cleaning itself up after the rebuild, I cleared it, and it threw it again after a couple miles. It could of course be a coincidence, but I'm curious now if they sent me the wrong injectors, and that is somehow causing this. I have a new set of O2 sensors on their way, but I'm wondering if I should be pursuing a new set of injectors as well.

 

Well, the o2 sensor was a coincidence, and not a happy one. Some rodent (or something) got under the car and chewed the wiring out. Now I'm going to have to figure that all out.