Convert 12V 200mA to 5V 450mA
You would do well to go to the TI web site and utilize their WebBench design evaluation tool to enter your requirements and see an optimized circuit (or selection of circuits) pop up. For sure you can achieve 80% efficiency or better with todays components. 

Hacking an existing design that:


You do not seem to understand how it works
You have not read the data sheet inside out
You fail to understand the functions of R2 and R3


...is not a way of going about designing switching power supplies. 

Also make damn sure you understand the implications of usage and safety when designing non-isolated off line power supplies.