Park Street is only about 1 in 12.
Does that mean about 8% ? That should be within most bikes and riders' capabilities ? Any Bosch or Impulse crank drive bike should really fly up there even more so with a little bit of wellie and a higher gear. It's interesting that the Ibex is getting reports of being good at assisting but only at such a slow speed. Have to admit I am very confused !
Still find all these torque figures confusing. I'd estimate newest incarnation of Impulse would assist to 15mph on an 8% with a rider being prepared to 'go for it' a bit (without killing themselves trying !). On a 1:4 or 1:5 it should be a good 7.5-8.5mph with 'relaxed flat riding' pedal force and 8-10mph with geared effort.
I also saw some wild claims on Aseako's website about hub motors in a comparative sense...
Central Drive System | ASEAKO Electric Bike
... but I note there is nothing said about sustainable speed when climbing, which may not matter to some people but matters a lot to me. Not sure where this fits in for the OP. It's one thing to be able to get assistance climbing but if you're doing 4mph it does beg the question whether you may as well be off the bike pushing it !
I can say with 1st hand experience a 500W CST hub motor is quite capable of assisting a totally sweat-free climb (light pedalling in a higher gear) on a 1:5 (20%) slope at a very decent pace. That's in the 10+mph range. I found a bike from linked range needed considerable effort on what can't have been more than a 12%, and was relatively slow even on that. So whatever statistics or claims are made about torque, if you want to climb hills on bikes the only way to see what does it for you to deliver torque in the way you want it for your riding (and at the speed you want to climb at) is to get on one and try for yourself. Other factors seem to affect what a bike delivers, how and when on a steep hill.
May be wrong but I suspect the controllers have a lot to do with it in many cases.
