I leveled shadowstep for a long time. It sucked, and I kept at it. I was killing slow, but I kept at it. Eventually I got tired of it and switched to Mutilate, and started leveling about 50% faster (that is not an exaggeration).
If you feel like you're going to pull your hair out with a faster leveling spec, don't spec it. If you honestly have tons of fun leveling as shadowstep, go for it. If you're like me and you completely zone out while farming/grinding and speed is the only factor, go for the best spec you can.
Yeah, combat swords is boring, but welcome to grinding. Everything is boring. I prefer to spend the least amount of time possible grinding and farming, then have fun when I'm at the level cap. I guess the FF XI grind scarred me for life (takes about six months of steady play for the average player to hit the max level), and I'm most concerned with getting to endgame content fast.
I've got a 70 Rogue and 70 Mage, working on a Paladin now, and my system works for me. If you just want 70 fast, Combat is the way to go. If you want a change of pace, Mutilate is right up there with it, and is tons of fun to play. Ever crit somebody for 4,600 damage out of stealth (shortest arena ever)? You might as Mutilate.