a used 5sp from a 5K donor car with unknown mileage... will it handle 275whp? probably. would it handle it reliably? define reliably. if a consequence of bolting a "reliable" part into my car is that i don't have to baby it, or change how i'd like to drive, then no. if i put a 5sp in there, i'd baby it, and i'd be worried about it. that's why i put a 6sp in my car. and that's also one of the tings keeping me from getting an 818 kit at the moment - they don't even have provisions for people who already have a 6sp.
the other thing about the 5sp/6sp topic is that, as mentioned, the car had humble goals. it's a light car so it doesn't need huge power and bulletproof parts to be fun. so they provided a solution for wrx donors to be faster than sti's with a much lower entry point. but maybe they underestimated the fact that subaru enthusiasts already exceed the performance levels of stock sti's, and that almost all car guys (the target market for this car) always want MORE. so since they did such a good job of exceeding the performance capabilities of an sti, then the sti guys want in on it too! and they want to transfer their better engineered sti parts that they already have, instead of downgrading to make the swap. evidence of these car guys who want MORE, is in your example of people with money to burn. i'm going to guess that those people would rather spend their money on a stock 6sp that can handle the power, instead of spending money reinforcing a weaker design.