At that power level, our standard SAE 4320 axles should suffice. We use these on our BMW 335i's putting down over 600lbs without issue.
300M is expensive and over-rated. There are better choices of steel available when you do start pushing 800lbs+ (which is highly unlikely in your Fiesta ST's)
Also, just to point out:
1) Whilst we would be more than happy to produce axles for you, we would not want to make them if they are not really required (yeah, I know. Contradicts business sense, but that's how we operate! We want to work "with" the community, not just make money off it)
2) Broken axles is not just about power, but also chassis setup and, most importantly, driving style. If you are breaking axles at relatively low power levels (and it's not a known fault/issue), then you would be best looking into other areas of your setup to see if that is the cause of the issue(s). Anything from suspension setup to driving environment can cause a failure i.e lots of variables