Yes, colder air will make it run a tad leaner, but not so drastically that it would cause the problems you are reporting.

Syncing the carbs is a mechanical process that when done correclty, syncronizes all four carbs to pull the same amount of vacuum at idle with the expectation that they will pull the same throughout the range of throttle.

Using mid-grade fuel is less than ideal, but this won't cause these problems either. Worst case is you will be down a little on power and the motor may knock a little when you lug it.

The solution is in the jetting. Taking the bike to someone with a dyno and O2 sensor can give you a graph of your hp/torque and a/f mixture. Looking at a/f throughout the rev range will show you where you are lean and a good dyno operator can give you suggestions on what to change to fix it.

Back in the day Dynojet used to give a certificate for a free dyno run when you purchase one of their kits. I used mine to verify I had my setup correct after I installed the kit.