I have tapped up to 1/2-13 with OSG thread formers all day long at 1000 rpm with no problem with a VF-2. Make sure you use the right starting hole diameter and gracious chamfer as the form tap will upset material upward. for instance recommended .460" start hole for 75% thread for 1/2-13. I use .470" (vs. .421" for regular tap)
osg website has a calculator for form tap hole sizes.
http://www.osgtool.com/Technical.asp?tid=1&id=1
I would form tap larger if the taps were available, hard to find larger than 1/2"
also i have thru tapped (normal gun-type tap) 3/4-10 into 1/4" mild steel at 500 rpm with a TM-2 no problem. single pass with slightly oversize tap hole.