I would think so as most people who visit MK stay for the day to fit as much in as possible. You will find because it's not on every evening, Spectro will always be very crowded.
When we when to see it we arrived on Main St about an hour early (this was in early June) and found a place to sit then just waited. In about half an hour Main Street was getting very full already.
Just prepare for the crowds and you should be fine!
I wouldn't say morning EMH would have much impact on crowds for the evening but, bear in mind that those guests who don't 'hop' will have to stay in the same park for the day so, if they choose to do the EMH in the morning, will more than likely be there for the evening too. Wherever possible, I would always avoid a park that has morning EMH though - but that's just me.
General rule seems to be that if you go for a morning EMH, hit it, hit it early and then around lunchtime, get yourself out of there! That being said, Spectro will always attract people and you can plan accordingly.
It is likely that the EMH people will have left and possibly come back later, I am sure it is not easy to advise.
What I would say is that if there is EMH evening hours and spectromagic - those are the ones to avoid. We found the evening times to be worse for getting on rides. The morning EMH are far better - maybe the USA tourists dont do early?
Thanks for your replies. Spectromagic seems to be on every night for the first week of our stay so we'll probably avoid it on the EMH days.
(I'm a bit nervous this year as we normally go at the end of August but we're going end of July/beginning of August this time - I hope I'm not too shocked by the crowd levels.)