Yes, the A12 gearbox is light duty, while the A14/A15 gearbox is heavy duty.

The A14/A15 gearbox and the B310 differential are strong enough for a street car, as long as you aren't doing full-throttle starts in 1st gear I don't think it will break even with a high-revving A15 at 150HP. If you intend to race it, then the diff should be upgraded to H165.

150HP Turbo can be done for $1000 and a lot of work. Start with a good A15, use stock block and head. No need to open up the engine or add heavy-duty parts (it doesn't need to rev very high). The key is in fabricating nice intake & exhaust and tuning correctly. If you you have to pay someone to fabricate the system yes it will $$$. Sykine acheived 150HP A12 with stock block, stock headgasket & stock A14 cylinder head. And very careful tuning. This will have much more torque than a naturually aspirated 150HP A15, so if turboing it upgrade both the gearbox and the differential.