Speaking about 170 performance, whilst they are slower off the mark than a 158 it's not a huge difference, they are totally interchangeable with 158s up here on commuter work. For example, the Fife circle is probably similar to the work they are doing in Yorkshire, with a brief spell of 100 (where you get to about 85), other than that you are pretty much 65 or so and they are fine with that. Not great with hills but neither are 158s. The transmission change at 70mph is where they really start to outperform a 158 though, that is true. They really aren't that bad, like everything they vary from unit to unit, the only niggly thing with them compared to a 158 is the delay between taking power and it actually doing what you want it to. A 158 responds instantly, a 170 has maybe a 3sec gap between you moving the throttle and the revs actually changing.