While I'd agree Italy was a net drain for Hilter, by and large (Greece did not join the allies til Italy delcared war on her, so the threat you are talking about from Greece was... less than you are projecting, if Italy was netrual) It's really hard to figure which was worse. One fact you all keep missing, is until 1943, Hilter was really pushing the logistics commitment of the Eastern front with what he had, meaning Rommel and company really weren't going to go to Russia. Mabye they could have replaced a Italian divison, mabye not. But overall, logistical supply situaiton in 1941 and 1942 pretty much did make those troops ... secondary to requirements, in a lot of ways (plus the Africa Corps did not have the best tanks, and vs. T-34's, what they had was... lunchmeat.) Japan... same story. The US was already waging a defacto war, and FDR was pushing hard for a reason, and in a lot of ways, the 6 months really didn't matter that much (it wasn't til late 1942 that we had any combat divisons really ready for combat, and even then... well, Rommel had a few good days agasint them.) Now, if the US could have stayed out for a year, that'd have been ... different, but realistically... ah. As for the Allied troops tied up in the Pacific, again, for almost the same reason, ironically, those were not going to siginficantly affect the European command. Mabye the Marines could have lopped off a few addtional islands pre Operation Neptune, but realistically, there was not that much extra manpower supportable.
Now, I do agree Italy provided less support than Japan. What most miss, is Japan drew away a LOT of the LANDING Craft for the Allies. (Read up on why Dragoon {Invasion of Southern France} was delayed as much as it was.) The battleships and Carriers invovled in the Pacific would not have been as effective in the Med or North Altantic (Battleships at this point were only good to squish bunkers, and realistically the Allies had plans for that). Carriers, espically in the NA would not have provided much, if any addtional airpower, when you balance logsitics.
Given that the Allied command did _not_ learn from various invasion plans the Marines and Pacific and South West Pacific command learned (to be fair, most of those lessons were learned in late 43 and early 44, which by the time Neptune (the acutal invasion) was well well avadanced) That's a null factor, and the tying up of allied Landing craft in the Pacific theater delayed Dragoon, and kept the Marines out (they'd likey replace some infantry divisons) who had written the book on naval invasions (Remember, only the US had developed even a working THEORY behind contested invasions, no one else had even consdiered it possible. And we had begun testing it. The Marines were brilliant there.)
Overall, Japan did more to help Germany than most belive, but for reasons most do not consider.
Italy was the worst ally of Germany, by this standard.
