I actually happen to like the bass boost of the se215 even for metal (let's say i'm borderline basshead ), but yeah, a less pronounced bass response might do wonders as well.
Anyway, as far as IEM upgrade goes, i think i'll be sticking with the se215 for quite some time, and then just jump into the custom bandwagon. Looks like many new companies with really fairly priced entry level customs are joining the buisness, like custom art with their music one and two and other companies, so, might as well give the custom path a try, in the future.
Must say, my forbiddem dream is to get either a earsonic velvet or shure se846, but both being so expensive, and me being a student without a job, it'll be really long before i'll be able to afford one of those :(.
My dream when it comes to IEM's is the SE535, I doubt I would ever need anything better than that :p