Mag rods are only 5-10 grams lighter. Instead of the engine, think of all the stuff that is attached to the crank, a 904 can save over 15hp in some cases just by reducing weight in internals compared to a 727.
Don't go overboard with the weight reduction. You already have a nice internally balanced engine. You'd see more gains and longer service life by taking a lot of measurements and making every cylinder as consistent as possible to the other. Measuring for pushrods properly, measuring every valve with those pushrods to get consistent lift for all 8, degreeing the cam, a timing chain tensioner is a good idea, lightly cleaning the casting slag out of the heads, super tuning the ignition curve, tuning fuel with an air/fuel gauge. There's lots of cheap stuff that makes power.
