I'm closing in on 30lbs loss now, and I'd guestimate a couple mph increase in cruising speed and much more climbing ability.
However, I have no idea how much is from weight loss, vs improved strength and endurance.
About the closest I could guess to your real question would be when I load my paniers with 30lbs of groceries; this does noticeably impact my off-the-line acceleration, but on the flats once at speed, it doesn't really seem much different. It also, of course, strongly impacts climbing.