The booster in '84 is a different booster than the later and earlier 930. 78-81 930 used a brake system w/out a rear circuit pressure limiting valve and the actual booster coefficient is 3 (has to do with the internals of the booster). Then in '82 Porsche decided to add a 55 bar pressure limiting device to the rear circuit, keeping the booster coeff. at 3. Then in '84 when the Carrera came along, they decided to use the same booster and pressure limiting valve as the Carrera. P/L valve is less at 33 bar and the coeff. is reduced at 2.22. Then in '85 changed it back to 55 bar valve and booster was back to 3 again!!!
Look at teh last chart on Bill Verburg's handy brake spec page
Brakes
So should you find you need to replace the booster itself, theoretically you should be able to use a Carrera booster since your 930 is an '84?