If using a “simple midi out” surely you’re changing the setting in the Apollo so the sound can be monitored, this isn’t going back in and round so there isn’t any project delay messing things up (if you’re not doing that and using the internal monitoring, it’s still kind of the same). When using External Instruments, we can now load the instrument like a VST, an instrument track, in the Apollo you will have this set to not send the synth inputs direct to the speakers but into the DAW. Now we can add all sorts of processing to the signal, in real time, no need to record the sound coming in as an audio track but.
In my experience (Cubase) different buffer sizes will change how accurate the audio is. If i remember rightly a large buffer ended up ahead of time, a small buffer behind but it could be the other way round, 512 or 256 was compensated close, without needing to do anything.
Here’s the solution… PUT VOLUME SHAPER ON THE MASTER/STEREO BUSS PURELY AS A VISUAL TOOL. I keep suggesting this and it widely gets ignored. Set it to 1/4, enlarge the view. Have some midi play a very transient sound off beat, now you can use the grid of VS to dial in the correct delay correction for the synth. No guessing. You will noticed slight forward backwards drift, that’s just the downside of MIDI