How much power is enough? 250 - 270 awkw is plenty for an exciting road car. If you can get 300kw from your set up you can stop right there.
For the track or drags there is never enough. The Stagea will never be a competetive circuit car and if you want to race regularly you would be better to build a dedicated track car. Because I am old and poor I am using my Stagea on the track just for fun and will get a 1/4 mile time just to see what it is but again it will never be a competitive racer.
If you do want to change your engine RB30 is the way to go. I recently saw a Tomei stroker kit going for 20K. A VL block is $200 or the real deal from Rips $5000. The head is not so important so long as you have a decent turbo. I have got a GT3540 which will take me as far as I want to go. I am using an RB25DE (n/a) head because it is cheap, has big ports and vvt. AN RB26 head would be better for obvious reasons but cost 10x as much.
if you want the sound of an RB26 screaming at 10,000 rev minutes (and it is exciting) you will be paying through the nose for the privilege. An RB30 will give you the same power with heaps of low down torque for a fraction of the price.