There are way too many variables in a question like this.. you're making generalizations. HEP pins are tailored towards their particular customer. HEP can restore a game for multiple customers, and do things differently toward that customer's request. So, yes, a HEP game carries a premium - but not to all buyers for every game. That's OK, though, as you'd find a buyer pretty easily if it's priced right.
Secondhand HEP games can be quite a bargain. I bought a HEP I500 years ago for about what half the original owner had in it - and it was still as beautiful as it was the day it left HEP - but the person who had it restored couldn't sell it for what he had invested in it.
Now, I500 isn't an 'A' title like a MM - on a MM, I expect he could've recouped a much higher percentage of his investment.
The problem is, a 2008 HEP TAF isn't restored as well as a 2012 HEP TAF - and it's 4 years older. A craftsman improves his craft daily. That's not a shot on anyone's work at all. Just a fact.
So, there are a lot of variables to consider.
Don't look at it as an investment. Look at it as your favorite game, restored to the hilt - and a game you wouldn't ever want to sell.
There are other restorers that rank just as high with me, but I don't want to open that can of worms. It wasn't part of the original question.