@gembo: "how much bad behaviour (e.g. forcing it over the 15.5mph) will the batttery take"
I'm not aware that riding at over the motor cutoff speed has any tendency to cause battery damage. Do you have any references for that assertion?
My understanding is that, at least on the Bosch system which is what IWRTAS' trial bike is fitted with, it mechanically disengages the drive from the motor if it senses that you are exceeding the cutoff speed. (Riding my Bosch-equipped eMTB at around 25kmph you sometimes hear a noise that sounds like a clutch mechanism hunting between being engaged and disengaged.) That also means that you're not fighting against an unpowered motor when pedalling the bike at speeds above 25kmph, or trying to push charge back in to the battery (which is what your post seems to be suggesting). The extra weight of motor + battery is still noticeable as soon as you hit a gradient, or try to accelerate smartly, though.
There does seem to be this idea that as soon as you reach 25kmph on an e-bike you hit a brick wall where the system actively works against you. That is absolutely not my experience of more than 12 months riding my eMTB. Pedalling at speeds significantly in excess of 25kmph on the flat is perfectly possible, though obviously not as much of a breeze as when you have a push from the motor at lower speeds. So you notice the loss of assistance, but only in the sense that you're then riding a heavy bike without the electric tailwind.
To IWRATS' point about turbo mode: I recently had the firmware on my Bosch system upgraded to the latest version and it feels like turbo mode - which was always "lively" - now acts almost like an on-off switch: if you put any effort in to the pedals at all then the motor kicks in at full chat, rather than the more subtle graduated input-torque-sensing assistance you get in the other modes. It's great for demolishing long, straight, boring hills but not at all easy to control if you're on nadgery ground like a switchback climb at a trail centre: put a little bit more oomph through the pedals at the steep apex of a tight hairpin bend - which is a fairly instinctive thing to do - and you're likely to find yourself heading rapidly towards the scenery.