well it more to it than .3 liters over. you also change the stroke of the motor and therefore make it perform better. the other thing is that when you make a 3.0 or 3.1L engine, you also do valve work, port the head in some cases, use stronger pistons, balance everything, do the exhaust, and intake work. in the end it is costly for all the extras. if you were to just get a crank and throw it in along with some longer rods and bigger pistons, it wouldn't cost that much at all, a little more than a rebuild. with a nicely built engine you can run NOS or high boost turbo, and then youll notice the power difference.
I've got a 3.0L in my 72 and had it on the dyno this spring. The 2.4L had an estimated 150 hp and 135 lbs of torgue at the crank. I have 260 hp and 278 lbs of torgue in the 3.0 (that's 209 hp and 256 lbs of torgue at the rear wheels at 5200 rpm) All this with a mild cam and enlarged SUs. Going to triples or a turbo set-up would give me another 45 to 150 hp. It would cost me another $1,000 to $3,000 however.