2011 Tacoma TRD Sport Double Cab Long Box
They already made a 3.0L H6 that fell within those figures...in 2005...with no direct injection.The first question depends on whether the goal is performance or a mainstream engine with good MPG that runs regular grade fuel. My assumption is that Subaru is going for the latter choice. 240 to 270 HP with a reasonable torque output of 210 to 250 lb-ft should be attainable with direct injection and without turbocharging.
Mine is 245 HP / 215 lb-ft of torque, and with something as simple as better headers and a tune you're up to the mid-high end of your range.
I don't see any reason why closer to 300 HP wouldn't be pretty easily attainable on regular fuel out of a 3.0L H6 today.