From Sheldon Brown:

"A note about capacity:
Manufacturers have to assume that their customers are clueless, and will expect the chain to have some tension on it even in the bad gears where the chain is using small chainrings with small rear sprockets. Thus, the rated chain-wrap capacity is very conservative. A competent cyclist who uses the gears properly can generally exceed this by several teeth with no problem.

Most derailers also specify a maximum size rear sprocket. This is often a pessimistic value, based on the largest sprocket that is normally provided as part of that group."

I think you will be fine in terms of the rear derailer taking up slack as long as you avoid the small-small gear combination. I am running 50-45-24 chainrings with a 14-28 freewheel (40T total) and a derailer rated for 33T capacity. This is no problem, I just have to avoid the 2 small cogs when I am in the 24 up front (chain starts rubbing on the middle chainring then anyways). The 105 will definitely be a nicer derailer than the alternative you present.