you do mean more overstable than a roc don't you? since there isn't much that's more stable(ie: neutral) than a roc. i don't think a spider is more overstable than a roc. if you're a dx roc thrower, i suggest keeping newish dx rocs around for more overstable shots. a champion roc would also be exactly what you're looking for, but a little pricey. a wasp, in my opinion, is not as overstable as a new dx roc or a champ roc. gremlin's feel so good in the hand, the best in the hand for a sidearm midrange shot (imo), but you have to search on ebay these days since they aren't making them anymore ($20 or so for the proline). dave just said on the pdga board that they'll make some STAR gremlins at some point, though.
if you really want a true overstable midrange, however, the champion gator freakin' rocks. there's no other midrange like it... very overstable with little glide. it's just an awsome overstable disc that you can really get some power behind. no glide, but fast and overstable means you can still rip it hard and fly a proper midrange distance.
i definitely think there's a lack of overstable midrange... most seem to be just plain stable (wasp, roc, cobra, coyote, shark, buzz, spider... all *about* the same to me). the gator, however is definitely an overstable midrange!
i run on lazer beans