Even as a hostile neutral the CP gains. The UK must buy off Italian neutrality almost to the degree it bought Italian belligerency, the prime export being coal to Italy, and the Italians can respect the blockade, Germany would prefer they not, but assuming they do we still gain a lot. no additional front to waste resources on likely quals what could be smufggled to the CP, A-H can attempt to interdict Entente shipping and likely the threat alone bleeds off enough resources to alter the RN blockade and forces arrayed against Germany by just enough to keep the naval war more interesting. Serbia is bottled up and overrun just a little sooner cascading into the Balkans and Aegean, Greece is less likely to be pushed into the Entente, Romania too, Bulgaria is more secure in being part of the CP, Russia faces more A-H efforts that Germany is not having to provide for so it has just that much more to devote to the West. All degrees of change, so the CPs can trade some draws for wins, some losses for draws and some wins for successes.
Italy might simple ride out the war without any of the profit but taking no losses. Post-war the economy might still flounder as all the other powers stumble, Italy still has many unresolved claims against A-H, designs on Albania, friction with the OE or Turkey, and points where it might get on the wrong side of Greece, France and/or the UK. The Fascist evolution might never spark, indeed Mussolini might stay a socialist firebrand. The UK should still back Italy into the 1930s as a counter weight to A-H and as a distraction for France, without the Mussolini rabble rousing Italy should be a good ersatz ally, its internal chaos enough to keep it toothless, but it should be sliding left, a social democratic government should take root in the benign economy and peaceful med, I doubt Italy will do that much better but it will be the most prosperous southern European state, a regional power and anchored in both Libya and Africa, parlaying its place into better alliance with Britain, a good third power spot or realigning with resurgent Germany if ever A-H and it can bury the hatchet. I might push forward the economic betterment by a decade, Italy has lots of suitors and its biggest market will be the European Customs union Germany builds, staying aloof but amenable should give Italy solid place to work from. For me this Italy might look familiar on surface but oddly unrecognizable.