3500lbs and 175kmh in itself, shouldn't need anything exciting for brake fluid. The kleen-flo dot4 should be plenty for that. I use that in my RX8 which is about the same weight with driver and similar speeds.
Things to consider with managing brakes:
What pads are you using?
How would you describe your bake application while on track? Stabby? Aggressive? Progressive?
How much ducting does the Focus have? Is it blocked? Can you clean up the air-flow at all without too much effort? (remove screens, grilles etc)
And so on...
Are you by-chance using a scanner to remove all the air and old fluid from the ABS pump(s)?
It's not uncommon to have a spongy pedal after a whole weekend of racing. Like others have mentioned, do a quick bleed process to ensure there is no air.
One of the bigger issues with DOT4, is moisture absorption. RBF600/660 are big offenders and the fluids (primarily 660) are intended for short term use before being flushed again. This is just something you need to keep in mind with DOT4 fluids.
For what it's worth, I use:
Amsoil Dominator 600 in the Mustang (4200lbs, and requires DOT4 Fluid by Ford)
Motul RBF600 in RaceTrix2 (2300lbs)
Kleen-Flo DOT4 in the RX8 (3200lbs)
Why the variety? It's what was available where I was shopping when I remembered I needed fluid!