Weight at the moment isn't an issue - getting to minimum weight is pretty easy for everyone, I suspect.
Aero improvements can easily overcome a 5 hp difference, at least for everywhere but the slowest of corners - without bothering to do the calculations, I think that a 1% reduction in drag would do the job quite nicely, and 1% isn't all that hard to gain at this stage of the aero development we are seeing. Once the cars have been developed to the maximum that the rules allow, that may be another story, but not at the moment.
All that said. I believe that a 2 or 3 year engine rule could actually help more than hurt.