7 strings (and 8 strings) are totally unnecessary unless you need to play really high up on the highest string for solos and stuff. If you want the lower range, you can just tune down. I tune to drop Bb no problem. If you need to go much lower than C, a baritone is a better solution than a 7 or 8 string anyway. I've had a 30" scale baritone for almost 6 years now, it's great. Funny enough, I usually have it tuned to A, so, only one fret lower than the Bb on my guitars. I've done a few songs in drop G, it handles it no problem at all, could go lower with ease, but I like it to have more punch and low end to it, more like that middle ground between a guitar and a bass rather than just a low guitar.
I've never messed around with 7 or 8 strings, because making the neck wider really isn't anything I'm interested in. If I ever ran into being sad cause my dropped tuning taking away the highest frets on the highest string, I would just grab one of my other guitars that's only tuned down to D. Or even standard, whatever.
Ask yourself, is a wider and less manageable neck really worth an extra 5 frets worth of notes at the high end? If it's not, just tune down a 6 string and you'll be fine. People love 7 strings, but they're not for everybody and they're certainly not necessary. At the very least you can try tuning down your six string. Maybe you'll need to get your nut modded to handle the thicker strings (EJ21's are plenty heavy enough for tuning down to drop Bb) or you might not even need to. You'll for sure need to check your intonation (probably will need to adjust it) and you may need to adjust the truss rod too, but maybe not. Just check it.