The modification is not needed. I chose to do it.
Metz warns against using rechargeables in the battery pack. I used them with no issues whatsoever. Of course, they would like to sell you their expensive pack and dedicated charger

So long story short, use rechargables in the battery basket and be done with it. You might loose some of the maximum output, but that shouldn't be an issue. The power on that thing is huge even by today's standards.
Or buy dedicated rechargables/charger from Metz, but they are quite expensive.