B310 comes with H150 rear axle assembly. In my opinion it will be strong enough for a 150HP naturally aspirated A15. But not strong enough for a 150HP 1.8 liter.

Why change the crankshaft? A15 crank is very strong.

To make 150HP the camshaft will have to be changed (unless using a turbo). To make 150HP you'll need to rev to 8500 RPM, and the stock camshaft peaks at either 5600 or 6000 (depending on which year A15). With peak torque at 7000 RPM, the torque will be relatively low.