Honestly, I don't think IT Managers should get bonuses for uptime — uptime is part of our base job, and if we don't do that we shouldn't have the job. Even then, sometimes things happen outside of our control that pull this down, but that just makes it worse in terms of bonuses: we're rewarded or not for things we can't control.
I'm a little more sympathetic for support, but supporting users is really the same thing and it's just so easy to game. A manager who knows that's where his bonus comes from will have a tendency to create lots of simple tickets so that he meets his metric.
Really, when you're thinking about bonuses, you have to look at the projects. These are the areas where an IT Manager gets to be creative and show that he's worth not only what you pay him, but the bonus as well.
To evaluate projects objectively, you need to look at outcomes vs plans. For example, for next year you might plan a thin-client rollout in an area. The goals for this project are to reduce energy use and simplify management of those desktops, at a net savings of $X per year and initial cost of $Y.
To determine bonus level you might then use a formula like for X*5 - Y as total savings over a five year life of the installation, and put as much as 1/3 of that amount on the table as part of that manager's bonus for the year. Other projects would contribute other parts, and you're realizing potential savings from this project that will last longer than just the five year life of the initial installation because this service level is the new baseline for that area for the next deployment as well.
You then measure the actual costs of the implementation, actual management costs after completion, and actual energy use before and after to see how the manager did relative to the expectations for the project.
By contrast, a similar project in another area that just updated the PCs with this years equivalent model on the normal replacement cycle would not have any bonus impact, because that's just part of the normal function for that position. You want to use bonus money to reward creative work that helps the bottom line of the business.