I will assume you have an RB powered car, in which case I will say both the GTRS and 5558 are poor options for 300kw.
Firstly the GTRS is not capable of getting you there, that is a 250kw job and thats it. The Precision is a high boost application and will need E85 to make it a comfortable result.
There are better/easier methods of hitting the 300 mark on an RB (assume RB25 from here), I would recommend you look at a GT30 based turbo or a hypergear G3 unit. The GT30 has great resale and is a well proven item, it will need a custom dump/lines/intake. The hypergear G3 will need the custom intake pipe but will work with most of the stock lines and will bolt up to the OEM dump. The G3 is probably a little more likely to hit 300kw on 98 but the GT30 will be a little more responsive (both run a .82 IW housing).
If you want to move to an externally gated option you can pick up one of the 'steampipe' manifolds for a good price and look at a kando T67 for you 300kw mark. That will be more likely to net 330kw, and the mentioned kit totals roughly the same as a hypergear and much less than the said GT30.
Now back to precision turbos.