I don't think you'll see too much of a downside of going a few cm shorter. A few things to consider are the camber style like Wasatch already mentioned, as well as if the tips are blunted / effective running length.
For example, I have a 157 Banana Magic that I love riding. Came across a good deal on a '12 NS Evo 152 and I thought... what the hell I'll try it out and see if I like it. I've ridden it 2 days so far and even with being a shorter, more flexible board... I can still charge down the mountain.
No, it isn't as stable as the stiffer, longer 157 Magic. But that actually helps in its own way as it is easier to maneuver, especially in tighter spaces. I can still go full speed, balls to the wall with the Evo. It chatters a little more than the Magic, but its nothing I can't handle.
SnoWolf recently got a 156 Gnu Rider's Choice (I think), which is smaller than his average board size of around 160 (can't remember specifics) and he also believes that you don't really lose too much, if anything, when going shorter with the new technology of these boards.
Last comment, if you're riding mostly in MI... I'd go with a smaller board. I moved from MI to CO just a year ago, and there was never even a time in MI (Southeast MI = Holly, Brighton, Alpine, etc) where I could even come close to fully utilizing the Magic for what it was intended for. 153 Trice or even a 153-ish, more flexible board, would be great for just about all MI hills.