This sounds alot like brushless vs. brushed electric motors the more I think about it. ROAR approved brushless motor systems because there was demand, and a few manufacturers made commitments to produce them, and conform to the same motor specs. In the end, the LRP and Novak brushless systems are more expensive than a decent mod motor and esc combo, but not rediculously expensive. Take into account the high cost of maintaining and replacing a hot brushed mod motor, and brushless setup ends up being FAR cheaper in the long run.
Carb vs injection system likely would be similar, especially if approved by ROAR, in that multiple manufacturers would have to sell them, and they couldn't be too outrageously expensive. Performance would be the same as a very well tuned carb, but since it wouldn't lean out from time to time, the engine should last longer between rebuilds, justifying the higher initial cost.
They key is keeping the cost realistically low (1.5 times the cost of a carb engine seems reasonable) and having multiple manufacturers produce evenly matched solutions.
HOWEVER, I doubt it will happen because engine manufacturers make LOTS of money from lean runs!
Damon Palm's 2011 Race Rigs (the ones that have all the guts in them):
Thump RC 4 stroke "GSC10"
X-Factory X-6 Squared
Traxxas Slash w/ STRC LCG conversion
THUMP RC -The only 4 stroke race conversion-