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Without getting too technical, assuming you are keeping the same wheels.
I reckon you would have a maximum of around 20mm to play with between your rim and wheel arch for spacers. Of-course this is just a guesstimate, and your best bet is to put a piece of wood across your wheel arch and measure the gap between the inside edge of the wood and outer edge of your rim. You would also have to take into account tyre rim as these can protrude, depending on tyre make.
Most people who lower their GTC's do it on springs and pretty much the lowest you will get is 30mm. Even so, after lowering you will still have at least a good inch between top of tyre and wheel arch, so you will not have any issues with speed bumps or rubbing.
This is a gtc lowered on 30mm H&R springs with the same wheels as yours to give you an idea of the end result.
https://gyazo.com/d887a2f8e8dfecbab92937408efa7bd8
I reckon you would have a maximum of around 20mm to play with between your rim and wheel arch for spacers. Of-course this is just a guesstimate, and your best bet is to put a piece of wood across your wheel arch and measure the gap between the inside edge of the wood and outer edge of your rim. You would also have to take into account tyre rim as these can protrude, depending on tyre make.
Most people who lower their GTC's do it on springs and pretty much the lowest you will get is 30mm. Even so, after lowering you will still have at least a good inch between top of tyre and wheel arch, so you will not have any issues with speed bumps or rubbing.
This is a gtc lowered on 30mm H&R springs with the same wheels as yours to give you an idea of the end result.
https://gyazo.com/d887a2f8e8dfecbab92937408efa7bd8