Originally Posted by user92626
If you could serve like that with dead balls from a hopper, how much better, mph, would it be with new balls? Guesstimate?
From personal experience, I'd think it makes a max. difference of 10-12 mph.
OP: While a farther throw usually does correlate to higher serve speeds, the two motions are different enough to perform very differently on each.
To gauge serve potential, I'd start trying to get as close to a purely flat serve as possible, and taking off 10-15 mph for a slice serve and 30 mph for a generic 50/50 pace/spin kicker.