I personally did the head gasket on my EJ25D.
I did my research, took my time doing the job, and carefully picked what parts I would use for the head gasket. I do think it can last longer than 150k miles.
I wouldn't want less power than the 2.5 engine, I think it could already use more power. I wouldn't want to put smaller tires on my outback, I purposely got the outback because its closer to an SUV than a car, but with better gas mileage. It did a great job in the snow we had here last year. I put about 10,000 miles on the engine since changing the head gaskets, and the engine still runs nice, although only time will truly tell.
Of course I am sure a lot depends on how well the head gasket job was preformed. I made sure to clean everything up, made sure everything was smooth and not warped, and I carefully installed everything. I also use the correct coolant, synthetic oil (although this is mainly so I don't have to change the oil as frequently and I feel a little safer starting the car in the winter time, or driving for extended time in the summer), and I'm going to change them when recommend.