From my experience the main driving points to double the purchase are:-
a) Induce mast bend to flatten the main and
b) increase forestay tension to improve pointing ability as the majority of Bavarias don’t come with an adjustable forestay.
From memory the standard set up is 12:1 so by adding the extra block that becomes 24:1. The existing backstay plate on the stern is generally adequate for the 12:1 purchase however when you look in the lazarette to see how it is actualy fitted most owners go “Hey up! that looks a bit weedy” so they upgrade it for a better fixing and peace of mind.
I doubled mine up earlier in the year without strengthening the backstay attachment and have not had any problems.
Saying that there was also no noticeable difference in pointing or the ability to flatten the main so took it off. I think it is only worth doing if you intend to do a bit of racing and want the maximum out of the rig. For general cruising the standard set up is adequate, just wind it up as tight as you can.