Is playing without a HUD a mistake?

I'm a winning 100NLHE reg. Never used any type of HUD or poker tool.

Eventually, I want to move up to mid stakes. Can you beat these games with just skill and no HUD, or is this very hard?

Tried downloading one a while ago, but all of them were bugged and I simply never bothered to look into it again, until now.

I do make a lot of notes on players.

Ivey and Ilildur don't use a HUD, but those guys are uber-geniuses, at Phil Laak would say.
Also, they play a very limited amount of opponents.