you can make a typ1 irs box hold up but have to spend the money.
know of a few people who push upwards of 350bhp through one with slicks and there still in one peace.dont get me wrong you can break them but i know of people who have broke pro-drag box's with 120bhp
iam putting 170bhp and slicks through my pro-comp type 1 irs with no problems yet and intend to put more than 250 through it next year.
a porsche box is not the be all and end all of it,first gear is a week point and its a lot cheaper to rebuild a type 1 box with all the right bits than a stock 901/915 with all the wrong gear ratios.
have you thought of trying a typ2 irs box?
cheers rob