Minimizing risk of structual damage when hanging heavy bag from ceiling using commercially-available mounting system
I wouldn't worry about doing structural damage to your joists with this mounting hardware.  Realistically the 100 pound heavy bag won't generate any more wear and tear on the joists than a 100 pound kid upstairs jumping rope would.  The shock absorber will even out some of the peaks and valleys of the dynamic loads.  

If you really want to spread the load to more joists, sure you could attach boards to spread the load to more joists.  I don't think it's necessary.  

I would probably use 2-1/2" x 1/4" lag screws with full depth 1/8" pilot holes to mount this - nothing larger, possibly something smaller.  That won't appreciably weaken the joists. 

The most important thing is to locate the joists accurately and hit them as close to dead center as possible.