When my MBP is 100% charged, it shows above 7 hours and then discounting will start in 10 minutes under 3 hours and in 2 hours is almost empy charge
Me either! Clean install will not let the battery to drain fast it will drain every 9 minutes of uses sometimes 6 minutes.
I think you should wait for Apple to release the SMC and OS update, They will fix and release it to everyone who got affected by upgrade.
Apple Store won't able to fix this issue as they don't have any resource that's relating to this case but surely Apple might know this issue. But if you can't wait try to change your current battery to new one it probably helps as i heard that small numbers of people reported that they resolved their battery life issue by changing the battery, I can't confirm if that will help but you could try.