How long are the screws? I have 60" 1/2-8 2 start and they whip a little when I'm at the end of travel, but I can run at 150ipm (600rpm) with them. I have a 40" screw on my Y axis and it doesn't whip at all, running at 190ipm (760rpm).
Your much better off going with 1/2-8 4 start or 1/2-10 5 start instead of 1". You'll won't need bigger motors, and you'll get better performance. 1/2" 4 or 5 start is almost twice as efficient as a 1" acme screw.