If you change youre turbo you will need to change afew other things. Mainly injectors and the ecu map or get utec/piggyback or some other e.m. You seem to have all the other supporting mods. Also for the price id get a fuel pump.
Bigger turbos will push more air at the same psi of smaller turbos. Thus 17psi on the stock td04 is pushing much less air than an 18g at 17psi. You get all kinds of benifits from this like lower egt's, more power......
you could port the td04.
thats good hp numbers for basicaly stage wrx. enjoy.