If you read it right the Turbo efficiency is 300+hp, which I thought meant the Turbo running at a very high PSI can put out more than 300 hp. It says at 6 PSI you will get a 56% increase in hp, which doesn't sound to bad.
As for not trusting it to fit the Celica engine, I think it will with a little work. The piping may be off a little, but that can be fixed. It says on the site that it fits the 1ZZ-FE engine which is what is in the GT.