I'm 5'7" with 30" inseam and have been riding an 800GS for a year or so with no problems. I have the low seat. I'm not completely flat-footed at a stop but that doesn't bother me.
I don't know about custom seats but I was considering adding some gel or other extra padding but can't do it since it adds .5 - 1" to seat height - something to consider.
I've seen this topic come up before and I recall there's quite a few inseam-challenged riders on the 800GS. I love the bike and have no regrets!