The best parts of using the 5th injector is not mixing fuel, not searching for E85 when on the road somewhere, no tank to keep checking, not taking up much space or adding much weight.
I bought an Aux fuel kit but had reservations about the safety of fuel in the manifold puddling but so far it seems it has not happened but I sold it and went with the AEM WMI kit and spend a huge amount of time building secure enclosures for the tank, pump and battery in the trunk well and below it, ran the line under the car above the fuel lines, etc...
Now I live a couple of miles from an E85 pump, race gas there as well, methanol in bulk for cheap 35 miles away....reconsidering all options and will decide if I need to make a change or not once tuning is done on the GT2560R if I get that far, getting really tempted to just swap in the Gen 2 GTX2860R now.
If I did not have around $10k into my different turbo setups I would just get a nicely done C39 and be happy but for $1k I can swap to the GTX which is far easier since a simple bolt on and I have no stock parts except DP and a 3" DP I built in 2014.