About 300D turbo power increase, stock kkk turbo is too small for anything, and you need to add intercooler. If you just add cooler, with stock turbo and adjusting the fuel injection pump you can get 180. Before doing anything, add cooler first! Hot intake air will kill cylinder head.
After fitting good cooler, you can make hybrid turbo to old body of stock one, fit bigger wheels inside etc to raise boost pressure and airflow. And then, most expensive thing, modify fuel injection pump to increase fuel feed. Stock pump will limit power under 200 hp and injection time is too long for high performance. Of course bigger exhaust. Nicest thing to all, no need to open engine/modify, etc. Stock will take almost anything. Transmission is also very strong in turbo d, and if you want more safety margin, you can add more clutch plates inside gearbox


If you do all things all the way as i described....easily 350hp. 500E will have hard time to catch up...record here now for street car is now 432hp/570nm or so.