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Well a lot of people on this site will tell you that 400 to the wheels without a built motor is somewhat pushing it. Many have blown their motors with 400 and even less in some cases. It depends on the motor. Some guys have way over 400 on stock blocks and are fine. But if by saying "safely" you mean no worries at all, you won't get that with any type of FI.
I've had my TT kit for almost 2yrs now, 36000 miles on the kit with over 400whp. See my other posts and you'll know what i did to add safety to my install and tune.