In theory, yes.
If Italy is in the CP from the start, that vastly improves the military situation for that side:
1) France has to commit forces in the Alps, thereby having less for stopping the initial German offensive. This does not necessarily means Schlieffen Plan working as intended, but puts the frontlines a lot closer to Paris with worst Entente losses when (or if) the Western Front goes static.
2) Austria has considerably more forces free to use against Serbia and Russia, which may mean either that Serbia fall earlier, or that the Russian offensive in Galicia is less successful. The Eastern Front is better for the CP.
3) The Serbian Army has nowhere to retreat to.
4) Slightly more Entente resources tied in colonial theaters, though there's almost no contest here - Italy won't be able to keep her colonies.
5) The Entente would probably not commit to Salonika or Gallipoli, which frees Ottoman divisions to use (primarily against Russia) - although avoiding Gallipoli would actually a boon for the Entente manpower-wise
6) The Med turns from an essentially Entente lake to a contested sea - the Entente has still a significant edge but the overall naval balance is not so ridicolously lopsided - Entente navies streched a little thinner may have some effects.
7) Bulgaria probably joins earlier, Romania stays neutral or might even join CP herself - more bad news for Serbia and Russia.
Overall, an earlier Russian collapse followed by CP military victory in the West by 1916 is a plausible outcome, even though by 1916, Italy would be suffering huge shortages herself, since she was heavily dependent on British shipping for critical resources like coal (and Italian economy had important ties with France). The latter point is part of why Italy did not do this IOTL -without near-guarantee of a quick CP victory, it was considered too risky. Another important part was the almost iron-willed refusal by Austria to concede anything meaningful of what Rome wanted in Tyrol or the Adriatic - some proposals were vaguely made (by Germany mostly) but Austria was ultimately adamant. Finally, the Italian public and many decision-maker hated the notional Austrian ally more the notional French rival - a feeling warmly reciprocated by the Austrians - very few in Italy liked having to go to die for Vienna's interests, as that would have been perceived.