^ If by citing the above-mentioned "by-Hoyle" Subaru-literature, that such a recommendation is inherently superior then what we have as "community knowledge and know-how, I find that hard logic to follow.
There have been, time-and-again, errors in the Subaru "official" literature, as well as ill-conceived/executed products - for which we as a community have only come to realize specifically due to the knowledge and experiences of our fellow enthusiasts and hobbyists - the "racers, mechanics, and pellets."
This is why I don't particularly take exception to those who refuse to use the Conditioner in their vehicle - as long as they can provide good reason (and to-note, there now have been people who've torn-down their blocks and undertaken drastic re-builds of their late-generation Subarus, after many tens of thousands of miles and years WITHOUT the use of the Conditioner, that notice no issues, upon detailed examination, with either their cooling system or headgasket).
To me, until we've got more hard-data, it's a toss-up, particularly given the sheer number of highly-experienced Subaru professionals as well as enthusiasts/hobbyists that recommend specifically against the use of this product.
My car is a non-turbo car with lower-strain/etc and a potential radiator leak. The 2.5 Legacy's in the States are also non-turbo? (I am guessing on the second bit).
^ It depends on the model - we've got both turbo and non-turbo 2.5L variants, in the Legacy platform for both of the latest two generations.
Subaru's official statements, again, recommends the use of their Conditioner product for *ALL* Subaru vehicles, including the turbocharged variants.
Me, personally? Yes, I use the Conditioner in my '05 Legacy 2.5GT, which is, indeed, turbocharged.