Any 930 should be capable of 13mpg in town and 21mpg on the highway. If you are not getting those numbers get to a dyno or LM-1 and find the problem. Most likely a WUR adjustment is in order.
EFI will likely not significantly do any better as far as mileage is concerned.
RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8