Do you think the fact Sherlock chooses to shake John's hand is particularly significant? Like maybe he was trying to tell John "I'm actually high as a kite right now."

Nonny, it’s because if he’s going to die, he wants to ensure John is one of the last people he will touch. Given what we know now about the scene (ie. planning to overdose and die before he landed), this makes it even sadder. He needed to feel his skin against his own, and he needed to know what it was like.

Plus:

It was the closest to a kiss he was going to get from John at that moment.

He never intended to make it to his mission alive, and John was the memory he wanted to die with.