Sion's shield has 50% less health scaling than this proposal, as well as a lower base value. Yes, Sion can stack health, but his W passive probably isn't going to make up for 50% lower scaling.
Sion's shield also almost triple the cooldown at max level (13 seconds vs. 5 seconds). If his shield cooldown were increased, your proposed numbers start to sound a little more reasonable.
Still, another really noteworthy difference is that spamming Sion's shield in lane/jungle is extremely mana intensive. Sion's shield costs a huge chunk of mana even at rank 1. Using it repeatedly in trades will quickly oom him. Since shen is energy based, he can spam his shield to his heart's content in lane.
The difference in laning is really important to mention here. I think one of the main reasons Riot has been hesitant to buff Shen is because he is a massive lane bully against other melees. His laning power comes from a lot of factors that lack dynamic counterplay. He bullies other melees in lane because he has amazing base stats, he's energy based, he has innate sustain, he has spammable point and click poke, and he has a spammable shield. These factors allow him to beat most other melees through sheer attrition. Aside from dodging his taunt, you can't really "outplay" shen in lane. He's just a brick wall who constantly whittles you down while shielding and sustaining through your damage.
I'm not saying Shen is OP in the slightest. I agree he could use a small buff. But, best case scenario, that buff would improve his late game team fight presence without buffing his already frustrating laning phase.
So, maybe increase the cooldown on his shield, and give it 7% health scaling? Or make the health scaling increase with rank, from 5-10%. This would buff his late game team fighting without touching his early laning phase as much--he's already plenty strong in that phase of the game.