Kelly, you are right. I even mentioned the TO4-E50 in my post above.
Mark, its not about the horsepower so much as keeping the turbo in its proper operating environment for the conditions you are going to be giving. You mentioned "300-400hp" in your post, but it seem that perhaps you dont have a specific target in mind. Thats fine, but you are not going to land much above 180hp unless you have a specific target and a plan to achieve it.
The stock hitachi you mention is a T3 turbine. It and the 2nd gen 13b it ran with were only rated at 200hp in the car - and only for short bursts. I did what you are thinking about doing. I ran the stock turbo to see what I would get. I was making a little over 200hp and the turbo lasted for about 70 flight hours. Looking at its turbo map, the impeller was running over 100k RPM. It's very surprising it lasted 70 hours. It probably only lasted that long because most of the time I was not running full power. The turbo imploded on my first long cross country. I was almost home and decided to let it loose a little. I was running about 30" MAP at 17.5k altitude. It lasted about 30 min doing that.
My advice, if you want to experiment with turbos, is to install a shut-off valve on the oil feed line to the turbo. I now have one in my installation. When the turbo goes it will be hemorrhaging oil into the exhaust. Without a shut-off valve, you will run out of oil before you can get very far. With the valve, you can continue to fly n/a as long as you need.
Play around with that spreadsheet I sent you. Make up numbers that are realistic for how much boost at what altitude you realistically want to fly, then find a turbo to fit those parameters. You can cut and paste various turbo maps onto the spreadsheet. They are available on the internet but most of the relevant ones are there on the spreadsheet. Try to keep the turbo in its center island of efficiency while staying away from the 'surge' line. Also, keep the RPM under 60k if possible. You will find that Kellys suggestion of the TO4-E50 is a good choice and readily available. You will also see that the stock T3 does not belong in an aircraft.
Best,
Dave Leonard