I realize you only did this for a quick test for the benefit of another TB member so no problem. If you do want to tune your five string Mikro EADGC or any other bass to any other tuning then there is NO long term concern about the tension on the neck AS LONG As you buy strings that are appropriate for the tuning you plan to use. With the right strings for the tuning you can put the total stress on the neck right where it was with the original strings and tuning. The neck won't know you changed anything and the truss rod won't either!
I'm glad to hear your initial report and any follow ons you might post. I have a slight notion to get a five string Artcore so I am interested to hear how the 5 string Ibanez short scale fives work out. It would be a matter of faith to assume that an Artcore plays as well as a Mikro but Ibanez seems pretty consistent to me and hopefully your Mikro results will match an Artcore's.
the artcores have a longer scale than the mikros - 30.3" vs 28.6" - and that's enough to make a difference. also the artcores are more expensive, and i expect they're probably better quality. my husband has an older artcore guitar - one of the gibson lawsuit models - and it's really nice.
btw, i'm a huge ibanez fan, and many of their basses are very consistent when it comes to quality. but of the two 4-string mikros i have now, they're completely different in quality. one has a super smooth finished neck, the other has a slightly rough unfinished neck. they sound very different, one being rich and the other mid-heavy and nasal. and oddly, the tuners are set on the headstock differently. they were bought at most a year apart.