I think one of the more annoying considerations in a deal like this is that we don't get to mix and match parts, for the same price- but that's life.
From reading threads on this forum, it seems that the OEM water pump is good for up to 200,000 miles (at least some of them). It's interesting to me that the water pump bearing has such a high probability of lasting that long when the idler bearings are dropping out between 80k-120k miles. Since there's comparatively little 200k miles data to date, I'm betting the OEM pump will last, so I bought the kit sans water pump. I have no indication from Subaru that they have not sourced parts from multiple vendors, or have switched vendors for this pump, so don't have a way of assessing the quality of what I'd be buying, OEM or otherwise, at this point. If I lose the water pump, c'est la vie- inconvenience- likewise the idlers, unless one ignores the noise they make when they start to wear. Throwing the belt, however, could cost the engine, at any time.
Given that the idlers are giving out before the belt, I'm (comparatively) impressed with the Mitsuboshi belt, not because no one else can make a long-lasting belt, but because for the most part, we know what we've got with the Mitsuboshi.
I get concerned when threads turn to arguing with each other as opposed to expressing our views to help the OP.
Go OEM, or go home? Spare me, little subaru trooper.