LOL at calling the Legend series "mid grade". I'm not saying this to be a jerk, but seriously, they're amongst the best rods made and St. Croix has made it possible to get that blank in virtually every handle imaginable. That's pretty awesome in my eyes. If you simply don't like the grip, or the reel seat doesn't fit your hand well, there are two other options built around the same blank. Why more manufacturers don't offer this option for high end rods is beyond me.
So, I haven't fished the Extreme or X, but I do own a current Legend Elite jig rod (7' MHF, I believe), and it's sensational. The sensitivity, balance (Chronarch), and feel of the rod are excellent. The quality of the cork is fantastic, though, had the X model been around at the time of purchase, I likely would have chosen that instead simply because I prefer a split cork grip for jig and T-rig fishing. The guides are by far the best I've ever owned. They are small, but they manage line well and seem pretty close to bulletproof. I would own an entire set of Elites and Xs if I only had the funds. And this isn't even taking into consideration that the warranty and customer service far surpass that of any other manufacturers.
Here's the thing. You're going to get a lot of opinions on here. Some folks are going to be die hard Loomis guys. Those are also stellar rods once you get into the $400 price point on up. Generally, rods on the high end come down to personal preferences. You aren't going to find rods at that price point that flat out suck. The best advice I can give you is to just get them in your hands if possible before making a purchase.