This is a difficult and subjective question, but I think there are several things that you should do in this situation. Companies employ this 360 degree feedback for a reason, and if the people you are speaking with are genuinely willing to actually do something about your feedback, I think it's worth pursuing the "telling the whole truth" option, with some caveats.
Firstly, you must find out whether anyone is prepared to take action based on your review. You need to speak directly with your interviewing manager and possibly with an HR rep before the review proper.
This would be a 1-on-1 meeting to go through exactly what you're saying in your question. That while you're willing to provide honest feedback, you're concerned about both repercussions and the fact that you don't think it's likely that substantial change will come out of the process.
Your goals for this pre-meeting should be:
- Get the manager to understand and buy in to your point of view - this guy is making your job difficult and you actually want something done about it, you aren't just stirring up trouble, and nor do you want to provide a 'fake positive' review. You don't have a grudge, you just want to have a less toxic work environment.
- Prepare the manager mentally for the review. Because he will now know what's coming, you don't have to stick to a single "this is an issue" in the meeting proper, but you will be able to cover more ground, be more honest and the interviewing manager, rather than going on the defensive, is more likely to understand and work with you.
- Giving the manager time to think of possible solutions like transfer, managerial training, sensitivity training, and/or getting HR involved.
- Provide the reviewing manager with your goal end-state for this situation. Do you want your manager gone, or would you be willing to work with him as long as he gets some training or agrees to work on aspects of his performance? Are you going to quit if the outcome is business as usual? If so, why have you waited so long to talk about this with a higher-up? You need to have the answers to these questions prepared.
If you don't get a positive response from the reviewing manager at this pre-meeting, walk away and decline to give feedback on your boss. If forced, provide a "fake positive" review. There's absolutely no point doing anything if nobody's interested in doing anything about it, and you may do a lot of damage. At this point I would continue to endure the behaviour, go to HR about it directly, or quit.
Assuming your pre-meeting goes well and you think the reviewing manager is receptive and willing to act on your feedback, attend the meeting proper. You should thoroughly prepare for this meeting by:
- Identifying the types of bad behaviour from your boss. They should be things things that he does often and that are obviously bad for business.
- Providing examples of each type of bad behaviour, either with emails or specific examples with dates, times, and names of other people that were present and are willing to corroborate your story. You should provide one egregious example of this behaviour, but be able to back it up with others if challenged.
- For each example, explain how this affected your work and what you would have expected from a manager instead
- Provide constructive thoughts on how you think this behaviour could be improved, including training, mentoring, etc.
- Write down and print out this information, but only a single copy for your use as a script.
- Be honest but constructive.
- Don't use angry, inflammatory or emotional language.
- Potentially provide some positive feedback on those things that your manager does do well (there has to be something, right?)
This way, you aren't going to be put on the spot in the meeting and you can stick to the script. If your reviewing manager defends the behaviour of your manager or otherwise becomes hostile, cut the meeting short. Just stop providing examples and skip to the positive bit. You can try to go over why you're doing this once again, but it rarely ends well.
The goal of this meeting is to convince the reviewing manager that this behaviour is real and that something needs to be done about it. I wouldn't leave the meeting without:
- An agreement that some action needs to be taken
- A date for the reviewing manager / HR to have decided on a course of action
- An expectation for what you can expect as feedback (e.g. "I'll email you to let you know that me and HR have agreed a course of action which will commence within 6 weeks, but obviously I can't provide specifics".)
After all this, I wouldn't expect immediate change. If your problem boss does start behaving more appropriately, make sure you thank him for doing so, but obviously make it subtle.