Why do you say that?
Tomioka makes a lot of Subaru turbos and the Sube guys only have great things to say about them. I haven't read of a single complaint so far.
I'm planning on big power (near/at 500), so I'm looking at strong turbos. Not very reliable to buy a turbo and run it near or at it's hp/boost limit. Seems like a better idea to get a stronger/slightly larger turbo and run well under its hp rating.
(Not trying to argue, just trying to really consider my options haha.)
So, let's explore this a little bit.
You said you want to make low 400s for a daily setting with the ability to turn it up to 500whp.
Not sure what your background is with high powered 4-bangers, but you want the smallest turbo that will reliably accomplish your goals.
Running a bigger turbo gives you headroom, but it also gives you a higher boost threshold and more lag. Its easy to fall into the rabbit hole and end up with a car that sucks to drive 95% of the time...I've done it.
Regarding Tomioka, they seem to be solid turbos. That said, the 475 can't match the airflow of the EQT despite being about the same size. On full kill, 37psi I think it made around 460whp, someone can correct me if I'm slightly off. Compare that to the EQT which will crack 500whp on 30psi by the end of the month, and can produce full boost at 2700rpm with excellent transient response.
The 600 is somewhat bigger, but no one really knows how much because their spec sheet is wrong. If you can't get accurate specs, don't expect anywhere near the support you will get from EQT. Also, if the 475 can't make 475whp on nuclear, don't expect the 600 to be a 600whp turbo. What it actually can make is unknown, so you'd be taking a real gamble by being an early adopter.
If you don't think the EQT is big enough, you might want to look into the EFR line. The EFR 7163 has been throwing down some really impressive results, but it's a very different price point.
Also consider fueling, tuning solution, etc. If you're not going to fully support a BIG turbo and choose to run one, you're getting the drawbacks without the dividends.
