UK diving though not blue water.
I cannot see myself using more than 8 inch on each side for wide angle and close up photography in UK and even in blue water, unless shooting something unusual or experiment different technique.
I do not think that additional arm, 5 or 8 inch will give you far better outcome. It is the position of your strobes and their output which makes the big difference.
Some examples I took in UK, all shot using 2 Nikonos SB105 on 8 inch arms:
- JDelage likes this