A few Q's for you:
What audio interface are you using (and are you using the same one when you report lower latency thru Logic)?
Do you get any better/different results when using the built-in audio?
Does this offer any help: http://support.apple.com/kb/HT5155
Many many pros are using MainStage live, I've worked with many of them and latency is typically not an issue for them - or just something they work with.
How do you define bad latency in MainStage and Logic? By looking at the numbers? The world is not that simple and - yes - there will always be latency. If you use MIDI, the output latency is what is important (because that's the only one controllable, the MIDI input latency is defined by the MIDI hardware); if you do audio the roundtrip latency is important. The numbers itself can not necessarily be trusted, especially with external audio hardware it is often wrong, because some drivers _lie_ about their latency to the system. The only real way to find out is by measuring it with an oscilloscope.
The main factor is the buffer size, which you can adjust depending on your need. I wouldn't just move it to the minimum and the driver latency also to the minimum, because it has an effect on the CPU.
I can tell you that at identical settings Logic _has_ no better latency than MainStage. Period.
Actually I think I might be ok with it. Turns out there was an adaptive limiter on out1-2 I thought was just reg limiter. For me unacceptable latency is any delay between me playing a note and it sounding through the speakers. Ive only just noticed there are numbers for latency on each track (i assume the out1-2 sums them in its channel strip number?). I seem to be ok with my TCElectronic Konnekt6 though my CPU is in the red zone with adding couple software instruments up. Will investigate guitar later though I suspect that'll probably push it too hard... That's at 64 samples with I/o buffer on.
I found an article on mix magazines website interviewing a tech for one of reznors tours and he was using 2xserve computers with a p I soundcard @32 samples with various channels gettin sent to and from the FOH. For future investments what kind of machine do you think could handle 3-5 guitar+bass channels and about as many software instruments and samplers (logic+NI plugs) without breaking 60 or 70% load. On CPU load, what do the people you work with usually consider a CPU limit?
Argl, yes, the Adaptive Limiter has an adjustable latency on the output it affects everything. MainStage has an option to show the latency for each channelstrip under the channelstrip, it's worth to check that.
The limit is around 100%, don't consider any number as bad or good. It is more a headroom for how much more you can add/play, but not where you are at.
The current Lady Gaga tour uses several instances of MainStage (all keyboards and the drummer use them), all run on MacBook Pros. MacMinis are also very common if you want a rack installation. That NIN is using an Xserve is new to me, it's the first time I've heard about using an Xserve with MainStage.