I like maxing my q for consistent trading, but e is hard to hit without a minion, but if you can its worth it, but if you are laning another tank build character, that 210 damage is not really worth 9 levels.
I would still follow DoctorAsawa's advice and max E first. Your opponent is typically going to have a lot of armor, so the magic damage will completely sidestep that, but maxing E will also allow you to poke from a very safe distance (you're practically always going to have minions around in lane, and jungle monsters have no magic resistance). You're thinking that your E will deal 210 damage, but if you poke with a minion it'll actually deal 315 damage, which is massive. To deal the same damage with your Q at the same rank (and the same level), you'd have to charge up your Q for at least 1 second, which has a ton of drawbacks (it's easier to avoid than an E, it takes more time, it leaves you vulnerable as you charge up, it costs more mana and has less range). On top of that, your Q scales with total AD, so your damage will increase naturally as you level up without having to rank it (as opposed to Roar of the Slayer). This means ranking up your E first will give you a much steeper increase in damage, since you can't hope to land fully charged Qs all the time (particularly since charging your Q prevents you from autoattacking, so the net damage increase you're getting is actually pretty small). Another big advantage with ranking E first is that, unlike Decimating Smash, its crowd control increases with rank (so at rank 5 you slow by 60% instead of 40%), which will give Sion significantly more sticking power in fights.
TL;DR Ranking E first instead of Q is generally safer, more mana-efficient, more damaging (even in the jungle), and more useful. Try it out, and you'll see that it'll help a lot with your mana problems, and perhaps even improve you laning/early jungling experience.
Sion's current stats doesn't need any buffs. As a Sion main "Hetsugane" in PH, I can confidently say that Sion can overwhelm top lane. You just need a cooperative jungler to take the free kill because Sion on his own can limit an opponent's movement a lot. I suggest building Liandry's Torment, Sunfire Cape and Thornmail. In high elo matches, the only thing you need to be worried about is whether your teammates have decent play styles. And if your'e planning to play as Sion, you need a lot of "patience."