I posted this before and someone thought it was a good tip so I'll add it again.
To get those plastic Shimano bottom bracket cups really tight, first install the bottom bracket and hand tighten the steel side. Back it off a half turn or so then install the plastic cup and torque it down as tight as you think you can without cracking it (I haven't experimented enough to find the torque spec for this but with plastic, you can pretty easily feel when it's getting to that point). Once the plastic side is in, torque the steel side which will then push against the plastic cup from the inside, effectively loading the threads just like torquing it would, but with less risk of cracking the cup or stripping the splines.
YMMV but this has worked for me on the 2 bottom brackets like this that I have installed. I'll be installing another and hopefully will remember to check the torque I've been using on the plastic cup to make this tip a little more useful.