Hi Sam! Since this cake has such a low amount of sugar relative to its flour, I would be weary about making significant reductions to the sugar. Start with 10% less sugar and if you like the result you can go down to 20% but since this is a coffee cake unless the sugar is coming from another ingredient I would be afraid it would get too bitter below a 20% reduction.

As for streusel topping, you can go ahead and reduce the sugar by 25% without any significant side affects and only slight textural differences (less crunch) in reductions up to 50%. 

I hope you find this helpful!

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