If that's the standard aircon set up, I'm astonished at the vulnerability, It needs adapters to lift the unions and pipes. protection is all well and good, but anything bolted between the chassis rails may cause the rails to distort if it impacts anything substantial, so it's really about measured risk. There are companies out there who specialise in fitting aircon to cars and I'm sure that they could find a solution.
e.g.
http://www.claytoncc.co.uk/index.php?option=com_frontpage&Itemid=1I fitted an undertray to my Roadster mainly to limit the amount of road dirt which comes at the engine, and to reduce the chances of anything tangling with the alternator drive belt which is exposed. It's fitted in a way which will allow some distortion without affecting the chassis - penny washer clamps on the chassis edge, plus a couple of small through bolts. I then went on to fit an aluminium plate between the overrider brackets to protect the rack and gaiters from road dirt, and this mated up to the spoiler rather well. So this separates the airflow underneath - the spoiler limits this and deflects air at the radiator lower tank. It does run cooler in hot weather, but this is a judgement on the gauge position. Too cool is not a good thing anyway, and the Roadster doesn't seem to run hot, like the Plus8's do.
Anything in front of the radiator will reduce efficiency, including the radiator bars and the mesh grille - probably by 20%, but no grille = no protection from flying stones etc, so again it's a balance. Badges will add to this, and a full set will probably mask 10% of the rad and upset the airflow, but none of this is critical unless it causes a problem.
The sump will play a minor part of cooling, but only when at speed.
