Italy joins the Central powers
Italian entry would boost the Central Powers but also have some problems. Provided the French resist the initial assault, which they should still do albeit with heavier losses and probably more land occupied, they will still be a tough nut to crack and will probably be more on the defencive, at least initially.
The Austrians would have more troops after 1915, as their not facing Italian offensives
The two problems for the central powers would be:
a) Possibly the Ottomans join the allies, which would probably be a bigger boost than losing Italy. Forces tied down in the Caucasus, Suez/Palestine and Mesopotamia will be freed up. Will need some forces to operate against the Italian colonies but shouldn't require a lot. Since they have only recently occupied Libya it would be difficult to defend. Also there is no blockage of Russia so it would have much better access to supplies once the allies get their production act together. Also the idea of a Balkan front with an advance on Constantinople could easily draw in a lot of Central Power forces.
b) Italy has serious problems in terms of supplies. Very dependent on imports. Those by sea will largely be cut off and its ports and coastal communications are now very vulnerable. The allies will still have naval superiority in the Med., although it might be more stressed but if the Central Powers try and mix it they are likely to get really hurt. Also given relations between Italy and Austria I can see their fleets having problems operating together. Once the allies are able to establish sea control and a blockage the central powers have to supply Italy's needs as well as their historical ones.
If a) occurs I would expect the allies to still win, unless France falls in the initial attack. [May happen but still unlikely I suspect].
Steve