Would participating in Top Coder help me to get a HFT/HPC job?

If you want to become a better programmer, Top Coder is a great way to gain valuable practice in a competitive environment. Don't do it because it would look good on a resume, do it to further your own personal programming skills. Once you have those the job search gets easier.

TC doesn't lend itself to a deep understanding of performance issues.  If you want to do performance optimization I think your time would be much better spent by grabbing the NAS Parallel Benchmarks and seeing how much faster you can get one of them to run.  You might find yourself doing a deep dive into hardware performance counters, compiler optimizations, parallelization, or some exotic code reorganizations (and perhaps even hand-coding assembly).  Those are the skills that will get you hired.  A future employer is only going to see an indirect relationship between a TC ranking and your future success.  Being able to show a portfolio of codes you've optimized (and what you learned in the attempt) is going to make a much larger impression.

I can assure you it will not hurt your resume! Top coder is a very good training tool and proving ground. Many recruiters view it as the equivalent of a pro sport combine, where athletes are measure on speed, agility, strength ect.. Top coder is a ranking tool and that will get you noticed. Companies love two things about top coder participants. 1. They are not afraid to be measured! 2. They are competitive by nature! The prize money is a modest incentive the real Prize is owning a top slot on a global stage.