BTW, if you do go ahead with tuning the bike please do post your results and experience
There is already someone who derestricted a bosch speed motor, search for luv2ride.
This only makes sense if that tapering off between 26-28 bothers you too much.
And there are many who own a gen 2 bosch (I myself own 2) and derestricted them.
I am capable of riding at 30mph on flats, it still takes effort though. I have exceeded 35 however that requires a very serious effort unless it is downhill and not sustainable. For long periods, unless you are putting in the effort, even 28mph can be a stretch for these motors.
There is also a big misconception about peak vs nominal values. These motors will peak 600W+ only in bursts, they simply go back to their nomial after a short period of time. Very handy on start stops while climbing, mountain biking but doesn't make a difference in commuting/touring where you need a sustainable support. Bursts really don't mean much for long distance sustained effort(heck according to nyon even I can put 1000W+ myself for bursts).