Pretty much all books i have read about the normandy campaign cite this number +-5%. I remember reading in some book that this was based on a study done by the US Army in Normandy.

If this is true, is the importance of small arms over exaggerated?

Yes the importance of small arms is very exaggerated, this is especially true for rifles. More important than a small edge in performance in your rifles were the light machine guns. Machine guns have disproportional more effect than rifles. Having the superior light machine guns has significant more effect than having a superior rifle, it enables artillery fire by supressing-fire for instance.

The casualties due to artillery (particulary mortars) were high because the movement of american troops was very slow this enabled german troops to target the advances better. In the recent book from Rich Atkinson you get some detailed informations why german mortar and artillery teams inflicted serious damage despite their relative low numbers. For instance german soldiers would mark their own dug in positions into their maps, if the american troops forced the defenders out of their positions the german mortar teams would immediatly start shelling the fox holes which were already marked. The american soldiers would often occupy the fox holes and then get shelled. In contrast to the Wehrmacht the US army had, at least in the beginning, not the same emphasis on digging in. The Wehrmacht was very experienced and learned a lot of lessons, this included to always dig in even if only for some hours. GIs often did not dig them self in properly and were exposed to artillery.

How does this compare to both the war at large and later wars

In World War 2 this mostly depends on the armies involved. If an army is vastly superior to its enemy either due to efficiency or numbers or both than the enemy casualties are mostly POWs. If the attacking army is not able to advance fast then most casualties will be artillery ( same like in WW 1 ). Casualties due to bullets will be the highest during urban warfare ( Stalingrad for example, Kharkov street fighting... ). If a battle is rather long with low intensity than non battle casualties like frost bite or battle fatigue will have a higher percentage. On a very open battlefield like the russian steppe, the casualties by AFV and machine guns will have a pretty high percentage. Same is true for desert. In north africa the gunshot wounds had a higher percentage than in western europe for instance. When your troops are under siedge more people will die due to starvation for example.