^^ Maybe you just suck at tranny swaps.. or driving.
There are PLENTY of WRX STi's out there making way more than 400whp and not chewing up rear diffs.
And how do you know the stock 5MT won't handle 550whp? i only know of a handful 5MT's that have granaded and over half of those cases were drivers doing stupid human tricks.
2005 Outback FMT