Just the initial shock during the initial phase of the war may be decisive if the artillery is employed well. Remember the Germans have very advanced chemical industry at the iime.
I am not sure how it will be deployed but let's assume a rather modest number of one regiment per army
1 rocket artillery regiment = 3x 4 launchers = 12 launchers x 5 = 60 launchers
4 launchers are equal to 72 guns so 60 guns = 1080 guns equivalent
Unless you can run, really fast after firing. early modern rockets are not a good thing. even the much later Katyusha was quite vulnerable to counterbattery fire (it leave ssuch a nice smoke trail to triangulate for location.
Your math is also somewhat in error.
Let's use the 82mm Katyusha (which was the result of years of effort) to fire 16 rounds requires 50 (as in FIVE ZERO) minutes to reload. It had a range of 6.500 yards and threw a warhead weighing 1.4 pounds/0.64kg. The famed French "75" fired three AIMED rounds a minute without causing the barrel to overheat (e.g. 150 rounds in 50 minutes) and well trained crew could close to put out 15 rounds per rate for a couple "mad minutes". The 75 had an aimed range of 7,400-9,300 yards (depending on shell type) with later war rounds taking this up to 11,000 yards, with a shell weight of 12 pounds/5.4 kg to 16pound/7.24kg.
To summarize a single 75mm can fire as many rounds rounds in 1:05 as a 16 round launcher can in 50 minutes. In 50 minutes the one gun delivers as many rounds as nine 16 round rocket launchers. Each shell weights between 8.4 and 11.4x that of a rocket warhead. This gives the total delivered weight in the initial one minute burst from the rocket launcher of 22.4 pounds/10.2kg. The 75mm gun total delivered weight of fire is 180/81kg -192 pound/116kg. (As an aside a four gun 75mm battery can fire
17,000 lead balls into a 100x400 meter space in one minute.
There will be shock and awe. It just won't be from the rocket launchers.
3-4 min reload time per launcher.
M-13 warhead is 4.9 kg 132 mm
You CAN NOT compare a modern MLRS to a WW I Weapon.
Might as well compare a Gotha G.IV with its 500 kg of bombs to a B-1B and its 50,000 bomb load. After all, they are both airplanes.