My speedmaster springs have lost about 600 rpm before float. But I have a solid cam and shift at about 7000 rpm. That's about where they float now. Not saying springs are bad, just not for my combo.
Most lose 9-14 lbs after heat cycling in a running engine turning upwards of 65-7000rpm.
Better the manufacturing..the less they lose. But most major brands 'Well known' and widely used stuff are light after 1000-1500 miles. Again why you spend the extra or CHECK and shim. . . Be it racing on a dollar or a dime...be smart.