Hyper MPG (highway) will be almost identical. The city drive cycle may suffer 10% +- w/o auto stop.
The biggest "problem" isn't MPG. Since the IMA and alternator functions are integral the IMA battery will eventually require replacement to allow proper 12v operation. They 2 systems are interdependent and there have been credible reports in here that confirm the above.
Adding a conventional 12v chargins system won't be easy and even more so if your AC equipped (no location for alternator mounting).
HTH!
