You have never had the experience of escorting on the Trinity map from the beginning field locations have you?
Whats worse is ignoring the contribution of the P-51s that escorted the B-29s over Japan. From November of 1944 to August of 1945 aircraft flew into Japan and you feel justified in saying that this was a relatively unimportant part of the war (the implication from your assertion)? I dont believe the development time would be all that significant but certainly providing for more accurate and precise elements that actually did see use in the war for the aircraft we already have is a valid expense of time.
From what Dan said, P-51s didn't use those drop tanks to escort B-29s, they only used them when carrying rockets because the rockets increased drag and thus fuel consumption.
You are asking for them for a reason they were never used for in reality. You are also totally blowing the situation out of proportion.
Try using WWII escort tactics. P-51s did not escort bombers, of any type, from wheels up to wheels down on deep raids. They traded off, first with the Spits or 47s, then later in the flight with other 51s that had cruised in with full tanks and lastly with the Spits or 47s again.