Well, you are mostly right. But heavier rails are necessary for higher speeds as well. For example, now IR operates a lot of CC+8+2T freights with 22.82 ton axle loads. On 52Kg tracks, these heavy freights cannot run beyond 75kmph. An upgrade to 60kg allows them to be run at 100kmph. And similarly 60Kg rails cannot handle 30 ton freights (to be run on the DFC's) at 75kmph, so heavier or stronger rails are required for the DFC to allow both heavy loading and higher speeds.
The curves, PSR's, TSR's etc are equally important, but then this is a trunk route, so unlike branch lines or freight-exclusive routes, such issues are handled regularly and the trunk routes...
more... see regular upgrades for improving their traffic throughput. Yes, these too have to be taken care of before the 150kmph upgrade for the route, but it is going to be a much easier task (and already in varying degrees of progress) for a trunk route.