I still cannot decide if I should apply the original stickers or not. He's an extremely minty headmaster that I paid $85./shipped for last year.

First I have to know what the sticker sheet could sell for. Not that that will help my decision. As for applying stickers I'm good..... I just don't know!  

Need opinions!

 

Are you ever planning to sell the Highbrow? If not, go for it - it's yours after all. If it's an investment thing, I'd say leave them off. But that's just me.

 

I was like that a year or so ago when I got excellent condition complete Hardhead and Triggerhappy for a fiver each from a car boot sale and they came with their unapplied sticker sheets. In the end I left the stickers off. Maybe sometime in the future I'll just Reprolabel them.

 

I'd go with keeping the original sticker sheet intact and Reprolabel'ing Highbrow. He's out of box anyhow, applying / not applying the sticker sheet won't really influence the total value much at all. If you do label him, make sure you place them on neatly, with clean hands and tweezers.
Regarding the value, I'd say the original sticker sheet would be worth slightly more than the Reprolabels sheet, but not much more. Strictly something for the hard core collector; and Reprolabels has pretty much filled in the hole for high quality sticker sheets so your market will be very small indeed,