I agree, set your nitrous cut off at least 400 rpms before your rev limiter, more is better, basically you can set it where the car stops making good power, or a few hundred RPM after that.
I start mine at 3,000, but I am on a zetec and the svt's are a bit peakier.
The auto tranny idea??? Not sure, I know my nitrous set-ups, but know almost nothing about the auto on the focus...I have seen several Ford trucks even have major tranny problems from running larger tires as the computer has issues. For that you would need someone that knows these trannies well, I think sublime has done his research on that more than I have.
Speed is only a matter of money; how fast do you want to go?