Sions passive is.... well he basiclly dosnt have one in this META. Casualy walking away after he dies is all it takes. ANY CC will stop him form being a threat, just a 20% slow will mean he wont catch another champ.
As for mana, you can get the mana regen mastery. I dont play him without it. Another thing is
this is a legit starting item for him. Someone told me to try it in one match, and when I was up against an Ekko in the top lane I knew I was in for some serious poke. I got a D. Ring and kicked minions at him all day. He still has an easier time managing mana than Sion, but his low health pool ment I could drive him off with just about 4 Es and Sions E and W scale on AP, so its not bad.
Eventualy building into a
with the mana regen masteries also helps a lot. Also always build a
always, always.
My typical start is 2 reju beads, but in a high poke lane like when the enemy takes an ADC or Gnar, I get the D ring and a pot. I have 2 mastery pages for Sion. One general, tankin affair. The other Is damage focused with TLD, but the BOTH have mana regen mastery.
My only real complain about Sion is hes punished VERY BADLY for having a stacking health passive by have a pitifully low starting health for a tank. Still his win % isnt bad, hes getting by. The main reason hes not very popular is because there are straight OP options for the top lane and his kit is slow and not for everyone. It takes special timing to land his full combo and it takes more work than some are willing to put in. The other thing that has him seeing less play is that flat move speed counters his kit real hard. Walking out of his Q fast means he gets to charge it less, easier time dodging long range Es, easier escapes form W explosions, easier time side-stepping his ult, and easier time avoiding his passive. Flat move speed is very available in this META.
So I also consider myself a Sion main, but I dont think they will do any buffs based on his mana. Its manageable, and if its buffed, he will be too strong.
Also I have little problem chasing people. I always buy
on him, and if you can clip someone with your E they have to flash or dash to get away from me chaining a Q slow to it. At that point Im closing in for Furror MS procs and about to put you on lockdown. Dont get me started on his Ult for chasing.
Anyway at a 52% win rate, he probably doesnt need help really.