well i think isuzu was just conservative on power output. if you like the idea of a "kit" go for the GM, its a good engine overall. but i dont think the MPG is there.
also, heres a new one for ya,,, check out the common rail diesel developed for jeep. they claim 215HP and 376 FT LB from a 3 litre and mid 20's MPG, dont know much about it but conversions are available for FJ cruiser and most imported cruisers and they are emphasizing 80 series. site doesnt provide much info but you can check'em out, maybe e-mail them or something..
http://www.dieseltoyz.com/