One of my calipers is sticking so it's time for an upgrade!

I have black diamond discs and green stuff pads apart from that the car is standard. Even with those I think the brakes still suck.

I was looking at the 300mm upgrade that pumaspeed do but they told me it will not fit under the 16s without lots of work.

I was planning on upgrading the fronts and changing the rears to discs too but after searching around on the net I'm just confused about the best way to go.

Can anyone offer some advice please? is there any good guides on line somewhere?