One thing you're not asking - when you have an incident, how long is acceptable until someone actually looks at it? And will this impact your regular maintenance windows?
I saw a shop go from exclusively 8/5 to 50% 12/5 and 50% 24/6 over a gradual process (a couple years) without adding staff. The new issues created by this were somewhat mitigated by getting more redundant hardware, better monitoring (especially alert delivery).
The real problem is that everyone had come to expect a large maintenance window during convenient hours of the day. This was handled by having more people work from 8am-8pm, more people scheduled to work Saturdays and Sundays, and three-day weekends being considered an opportunity do to maintenance that had previously been done after-hours on normal days.
Another problem is that everyone had come to expect that only unusual problems, extremely severe problems, or problems with a certain small handful of systems, needed to be handled during the middle of the night or the weekend. This was handled by never defining these requirements, so that a severely sleep-deprived individual could choose to ignore a particular issue if he was unable at the time, without fear of repercussions. (Although it could cost the company a great deal).
People became increasingly burnt out and disgruntled. But IT management was able to hide these problems from their management, looking like heroes able to do more work with less staff. Upper management never understood that the decreased maintenance windows for hardware maintenance were a serious problem, because IT management would not admit that this caused an issue.
The environment became increasingly pathological and I ended up leaving due to IT management's habit of lying about requirements and issues all the way up the chain. The last I heard, the remaining staff is feeling more burned-out, more ill-treated. People are losing their families becuase three-day weekends are considered a time to do extra work, and because of the expectation of working such long hours that their family is asleep when they return home. (only to do more work anyway).
However, one group started offering secret "comp days", i.e. if Saturday is scheduled for 18 hours of work, then Monday could be scheduled as a weekend day. This had to be done in secret for fear of management finding out that extra work requires extra people.
The company's profits are increasing, and the salary of the IT management is increasing.
Vacations were surprisingly not a problem due to careful scheduling and a diverse staff that did not share many religious holidays.
It seems like an overall success to me.