Autarch on a Jetbike isn't a bad idea. It is a bit of a gambit though.
Putting them with Shining Spears is a perfectly acceptable strategy for a speed based army, but the shining spear itself is disappointing to me. In my mind if you're going to get that close to a vehicle you'd be better off with melta weaponry than with a shining spear. If you're going against marines then using a reaper launcher isn't a half bad thought, but you really have to decide what your Shining Spear unit is going to be for. Are they going to be anti-infantry, anti-vehicle, or anti-MEQ. Using a Reaper Launcher and a Laser Lance is giving the Autarch two different roles. Which is fine if he's on his own (which I don't really recommend) but Shining Spears really don't suit both roles on a battlefield. And since you have to use all of the models in the unit for one purpose, it's best to be geared for that particular purpose. Otherwise you're sinking points that won't get utilized. They're also points put at risk since Shining Spears are a risky unit. Not bad, but either they do amazing things or they fail horribly.
If you're playing against marines quite a bit, never underestimate the power of Power Swords. Shining Spears do get bogged down in melee, and a power sword can help turn the tide.