I still think they would have had enough outright pace. Power doesn't make all that much difference in terms of lap time. Where they might have lost out is later in the season, when they had their awful run in qualifying - with a bit less up at the top end, they wouldn't have found themselves in a position to use their braking advantage to come through the field so quickly.
Then again, with Honda backing, assuming things stayed the same, might they have had more funds to be a bit more aggressive with development with the rest of the car? I know they had the budget to see out the season comfortably and with a sufficient development budget, but in a battle for the title and with more personnel and resources, they might have made more of the chassis.
Keep in mind they lost time and resources shoe-horning a Merc engine into a car designed around the Honda power-plant too, and presumably the car wasn't optimised for the Merc in other ways either (weight distribution, rear suspension geometry, exhausts and cooling design and thus the aero package, etc).
I think they'd have still won the war, assuming the Honda engine was reasonably reliable.