depends how much you want to spend and what power you want.
Next step from that is a replacement ecu, nistune or piggy back. That wont really give you any more power. Probably just improve response and be a heap smoother. Your fuel economy may also get better.
Next step is a bigger turbo but depending on your desired power you will need alot of other components to support it. Ranging from 4-8k depending on your choice of parts and labour by yourself or by a mechanic. Also might not be worth it if the car has done alot of kms may need rebuild first then your talking 7-10k for that. By that stage your telling yourself ive gone this far i might want a new plenum, manifold the list goes on.
I say go for a nistune or some other ecu and keep your hard earned cash unless your loaded have some fun then.